1911 Encyclopædia Britannica/Zoology

by Edwin Ray Lankester

ZOOLOGY (from Gr. ζῷον, a living thing, and λόγος, theory), that portion of biology (q.v.) which relates to animals, as distinguished from that portion (Botany) which is concerned with plants.

History

There is something almost pathetic in the childish wonder and delight with which mankind in its earlier phases of civilization gathered up and treasured stories of strange animals from distant lands or deep seas, such as are recorded in the Physiologus, in Albertus Magnus, and even at the present day in the popular treatises of Japan and China. That omnivorous universally credulous stage, which may be called the “legendary,” was succeeded by the age of collectors and travellers, when many of the strange stories believed in were actually demonstrated as true by the living or preserved trophies brought to Europe. The Change
in the
character
of the
science.possibility of verification established verification as a habit; and the collecting of things, instead of the accumulating of reports, developed a new faculty of minute observation. The early collectors of natural curiosities were the founders of zoological science, and to this day the naturalist traveller and his correlative, the museum curator and systematists, play a most important part in the progress of zoology. Indeed, the historical and present importance of this aspect or branch of zoological science is so great that the name “zoology” has until recently been associated entirely with it, to the exclusion of the study of minute anatomical structure and function which have been distinguished as anatomy and physiology. Anatomy and the study of animal mechanism, animal physics and animal chemistry, all of which form part of a true zoology, were excluded from the usual definition of the word by the mere accident that the zoologist had his museum but not his garden of living specimens as the botanist had,^[1] and, whilst the zoologist was thus deprived of the means of anatomical and physiological study—only later supplied by the method of preserving animal bodies in alcohol—the demands of medicine for a knowledge of the structure of the human animal brought into existence a separate and special study of human anatomy and physiology.

From these special studies of human structure the knowledge of the anatomy of animals has proceeded, the same investigator who had made himself acquainted with the structure of the human body desiring to compare with the standard given by human anatomy the structures of other animals. Thus comparative anatomy came into existence as a branch of inquiry apart from zoology, and it was only in the latter part of the 19th century that the limitation of the word “zoology” to a knowledge of animals which expressly excludes the consideration of their internal structure was rejected by the general consent of those concerned in the progress of science. It is now generally recognized that it is mere tautology to speak of zoology and comparative anatomy, and that museum naturalists must give attention as well to the inside as to the outside of animals.

Scientific zoology really started in the 16th century with the awakening of the new spirit of observation and exploration, but for a long time ran a separate course uninfluenced by the progress of the medical studies of anatomy and physiology. The active search for knowledge by means of observation and experiment found its natural home in the universities. Owing to the connexion of medicine with these seats of learning, it was natural that the study of the structure and functions of the human body and of the animals nearest to man should take root there; the spirit of inquiry which now for the first time became general showed itself in the anatomical schools of the Italian universities of the 16th century, and spread fifty years later to Oxford.

In the 17th century the lovers of the new philosophy, the investigators of nature by means of observation and experiment, banded themselves into academies or societies for mutual support and intercourse. The first founded of surviving European academies, the Academia Naturae Curiosorum (1651),^[2] especially confined itself to the description and illustration of the structure of plants and animals; eleven years later (1662) the Royal Society of London was incorporated by royal charter, having existed without a name or fixed organization for seventeen years previously (from 1645). A little later the Academy of Sciences of Paris was established by Louis XIV. The influence of these great academies of the 17th century on the progress of zoology was precisely to effect that bringing together of the museum-men and the physicians or anatomists which was needed for further development. Whilst the race of collectors and systematizers culminated in the latter part of the 18th century in Linnaeus, a new type of student made its appearance in such men as John Hunter and other anatomists, who, not satisfied with the superficial observations of the popular “zoologists,” set themselves to work to examine anatomically the whole animal kingdom, and to classify its members by aid of the results of such profound study. Under the influence of the touchstone of strict inquiry set on foot by the Royal Society, the marvels of witchcraft, sympathetic powders and other relics of medieval superstition disappeared like a mist before the sun, whilst accurate observations and demonstrations of a host of new wonders accumulated, amongst which were numerous contributions to the anatomy of animals, and none perhaps more noteworthy than the observations, made by the aid of microscopes constructed by himself, of Leeuwenhoek, the Dutch naturalist (1683), some of whose instruments were presented by him to the society.

It was not until the 19th century that the microscope, thus early applied by Leeuwenhoek, Malpighi, Hook and Swammerdam to the study of animal structure, was perfected as an instrument, and accomplished for zoology its final and most important service. The perfecting of the microscope led to a full comprehension of the great doctrine of cell-structure and the establishment of the facts—(1) that all organisms are either single corpuscles (so-called cells) of living material (microscopic animalcules, &c.) or are built up of an immense number of such units; (2) that all organisms begin their individual existence as a single unit or corpuscle of living substance, which multiplies by binary fission, the products growing in size and multiplying similarly by binary fission; and (3) that the life of a multicellular organism is the sum of the activities of the corpuscular units of which it consists, and that the processes of life must be studied in and their explanation obtained from an understanding of the chemical and physical changes which go on in each individual corpuscle or unit of living material or protoplasm.

Meanwhile the astronomical theories of development of the solar system from a gaseous condition to its present form, put forward by Kant and by Laplace, had impressed men’s minds with the conception of a general movement of spontaneous Ideas of development.progress or development in all nature. The science of geology came into existence, and the whole panorama of successive stages of the earth’s history, each with its distinct population of strange animals and plants, unlike those of the present day and simpler in proportion as they recede into the past, was revealed by Cuvier, Agassiz and others. The history of the crust of the earth was explained by Lyell as due to a process of slow development, in order to effect which he called in no cataclysmic agencies, no mysterious forces differing from those operating at the present day. Thus he carried on the narrative of orderly development from the point at which it was left by Kant and Laplace—explaining by reference to the ascertained laws of physics and chemistry the configuration of the earth, its mountains and seas, its igneous and its stratified rocks, just as the astronomers had explained by those same laws the evolution of the sun and planets from diffused gaseous matter of high temperature. The suggestion that living things must also be included in this great development was obvious.

The delay in the establishment of the doctrine of organic evolution was due, not to the ignorant and unobservant, but to the leaders of zoological and botanical science. Knowing the almost endless complexity of organic structures, realizing that man himself with all the mystery of his life and consciousness must be included in any explanation of the origin of living things, they preferred to regard living things as something apart from the rest of nature, specially cared for, specially created by a Divine Being. Thus it was that the so-called “Natur-philosophen”The Natur-philosophen. of the last decade of the 18th century, and their successors in the first quarter of the 19th, found few adherents among the working zoologists and botanists. Lamarck, Treviranus, Erasmus Darwin, Goethe, and Saint-Hilaire preached to deaf ears, for they advanced the theory that living beings had developed by a stow process of transmutation in successive generations from simpler ancestors, and in the beginning from simplest formless matter, without being able to demonstrate any existing mechanical causes by which such development must necessarily be brought about. They were met by the criticism that possibly such a development had taken place; but, as no one could show as a simple fact of observation that it had taken place, nor as a result of legitimate inference that it must have taken place, it was quite as likely that the past and present species of animals and plants had been separately created or individually brought into existence by unknown and inscrutable causes, and (it was held) the truly scientific man would refuse to occupy himself with such fancies, whilst ever continuing to concern himself with the observation and record of indisputable facts. The critics did well; for the “Natur-philosophen,” though right in their main conception, were premature.

It was reserved for Charles Darwin, in the year 1859, to place the whole theory of organic evolution on a new footing, and by his discovery of a mechanical cause actually existing and demonstrable by which organic evolution doctrineDarwin’s doctrine
of organic evolution. must be brought about, entirely to change the attitude in regard to it of even the most rigid exponents of the scientific method. Darwin succeeded in establishing the doctrine of organic evolution by the introduction into the web of the zoological and botanical sciences of a new science. The subject-matter of this new science, or branch of biological science, had been neglected: it did not form part of the studies of the collector and systematist, nor was it a branch of anatomy, nor of the physiology pursued by medical men, nor again was it included in the field of microscopy and the cell theory. The area of biological knowledge which Darwin was the first to subject to scientific method and to render, as it were, contributory to the great stream formed by the union of the various branches, is that which relates to the breeding of animals and plants, their congenital variations, and the transmission and perpetuation of those variations. This branch of biological science may be called thremmatology (θρἐμμα, “a thing bred”). Outside the scientific world an immense mass of observation and experiment had grown up in relation to this subject. From the earliest times the shepherd, the farmer, the horticulturist, and the “fancier” had for practical purposes made themselves acquainted with a number of biological laws, and successfully applied them without exciting more than an occasional notice from the academic students of biology. It is one of Darwin’s great merits to have made use of these observations and to have formulated their results to a large extent as the laws of variation and heredity. As the breeder selects a congenital variation which suits his requirements, and by breeding from the animals (or plants) exhibiting that variation obtains a new breed specially characterized by that variation, so in nature is there a selection amongst all the congenital variations of each generation of a species. This selection depends on the fact that more young are born than the natural provision of food will support. In consequence of this excess of births there is a struggle for existence and a survival of the fittest, and consequently an ever-present necessarily acting selection, which either maintains accurately the form of the species from generation to generation or leads to its modification in correspondence with changes in the surrounding circumstances which have relation to its fitness for success in the struggle for life.

Darwin’s introduction of thremmatology into the domain of scientific biology was accompanied by a new and special development of a branch of study which had previously been known as teleology, the study of the adaptation of organic structures to the service of the organisms in which they occur. It cannot be said that previously to Darwin there had been profoundNew development of teleology. study of teleology, but it had been the delight of a certain type of mind—that of the lovers of nature or naturalists par excellence, as they were sometimes termed—to watch the habits of living animals and plants, and to point out the remarkable ways in which the structure of each variety of organic life was adapted to the special circumstances of life of the variety or species. The astonishing colours and grotesque forms of some animals and plants which the museum zoologists gravely described without comment were shown by these observers of living nature to have their significance in the economy of the organism possessing them; and a general doctrine was recognized, to the effect that no part or structure of an organism is without definite use and adaptation, being designed by the Creator for the benefit of the creature to which it belongs, or else for the benefit, amusement or instruction of his highest creature—man. Teleology in this form of the doctrine of design was never very deeply rooted amongst scientific anatomists and systematists. It was considered permissible to speculate somewhat vaguely on the subject of the utility of this or that Startling variety of structure; but few attempts, though some of great importance, were made systematically to explain by observation and experiment the adaptation of organic structures to particular purposes in the case of the lower animals and plants. Teleology had, indeed, an important part in the development of physiology—the knowledge of the mechanism, the physical and chemical properties, of the parts of the body of man and the higher animals allied to him. But, as applied to lower and more obscure forms of life, teleology presented almost insurmountable difficulties; and consequently, in place of exact experiment and demonstration, the most reckless though ingenious assumptions were made as to the utility of the parts and organs of lower animals. Darwin’s theory had as one of its results the reformation and rehabilitation of teleology. According to that theory, every organ, every part, colour and peculiarity of an organism, must either be of benefit to that organism itself or have been so to its ancestors:^[3] no peculiarity of structure or general conformation, no habit or instinct in any organism, can be supposed to exist for the benefit or amusement of another organism, not even for the delectation of man himself. Necessarily, according to the theory of natural selection, structures either are present because they are selected as useful or because they are still inherited from ancestors to whom they were useful, though no longer useful to the existing representatives of those ancestors. Structures previously inexplicable were now explained as survivals from a past age, no longer useful though once of value. Every variety of form and colour was urgently and absolutely called upon to produce its title to existence either as an active useful agent or as a survival. Darwin himself spent a large part of the later years of his life in thus extending the new teleology.

The old doctrine of types, which was used by the philosophically minded zoologists (and botanists) of the first half of the 19th century as a ready means of explaining the failures and difficulties of the doctrine of design, fell into its proper place under the new dispensation. The adherence to type, the favourite conception of the transcendental morphologist, was seen to be nothing more than the expression of one of the laws of thremmatology, the persistence of hereditary transmission of ancestral characters, even when they have ceased to be significant or valuable in the struggle for existence, whilst the so-called evidences of design which was supposed to modify the limitations of types assigned to Himself by the Creator were seen to be adaptations due to the selection and intensification by selective breeding of fortuitous congenital variations, which happened to prove more useful than the many thousand other variations which did not survive in the struggle for existence.

Thus not only did Darwin’s theory give a new basis to the study of organic structure, but, whilst rendering the general theory of organic evolution equally acceptable and necessary, it explained the existence of low and simple forms of life as survivals Effect of Darwin’s theory
upon zoology.of the earliest ancestry of theory more highly complex forms, and revealed the classifications of the systematists as unconscious attempts to construct the genealogical tree or pedigree of plants and animals. Finally, it brought the simplest living matter or formless protoplasm before the mental vision as the starting point whence, by the operation of necessary mechanical causes, the highest forms have been evolved, and it rendered unavoidable the conclusion that this earliest living material was itself evolved by gradual processes, the result also of the known and recognized laws of physics and chemistry, from material which we should call not living. It abolished the conception of life as an entity above and beyond the common properties of matter, and led to the conviction that the marvellous and exceptional qualities of that which we call “living” matter are nothing more nor less than an exceptionally complicated development of those chemical and physical properties winch we recognize in a gradually ascending scale of evolution in the carbon compounds, containing nitrogen as well as oxygen, sulphur and hydrogen as constituent atoms of their enormous molecules. Thus mysticism was finally banished from the domain of biology, and zoology became one of the physical sciences—the science which seeks to arrange and discuss the phenomena of animal life and form, as the outcome of the operation of the laws of physics and chemistry.

A subdivision of zoology which was at one time in favour is simply into morphology and physiology, the study of form and structure on the one hand, and the study of the activities and functions of the forms and structures on the other. But a Scope of zoology.logical division like this is not necessarily conducive to the ascertainment and remembrance of the historical progress and present significance of the science. No such distinction of mental activities as that involved in the division of the study of animal life into morphology and physiology has ever really existed; the investigator of animal forms has never entirely ignored the functions of the forms studied by him, and the experimental inquirer into the functions and properties of animal tissues and organs has always taken very careful account of the forms of those tissues and organs. A more instructive subdivision must be one which corresponds to the separate currents of thought and mental preoccupation which have been historically manifested in western Europe in the gradual evolution of what is to-day the great river of zoological doctrine to which they have all been rendered contributory.

It must recognize the following five branches of zoological study:—

1. Morphography.— The work of the collector and systematist: exemplified by Linnaeus and his predecessors, by Cuvier, Agassiz, Haeckel.

2. Bionomics.— The lore of the farmer, gardener, sportsman, fancier and field-naturalist, including thremmatology, or the science of breeding, and the allied teleology, or science of organic adaptations: exemplified by the patriarch Jacob, the poet Virgil, Sprengel, Kirby and Spence, Wallace and Darwin.

3. Zoo-Dynamics, Zoo-Physics, Zoo-Chemistry.— The pursuit of the learned physician,—anatomy and physiology: exemplified by Harvey, Haller, Hunter, Johann Müller.

4. Plasmology.— The study of the ultimate corpuscles of living matter, their structure, development and properties, by the aid of the microscope; exemplified by Malpighi, Hook, Schwann, Kowalewsky.

5. Philosophical Zoology.— General conceptions with regard to the relations of living things (especially animals) to the universe, to man, and to the Creator, their origin and significance: exemplified in the writings of the philosophers of classical antiquity, and of Linnaeus, Goethe, Lamarck, Cuvier, Lyell, H. Spencer and Darwin.

It is unnecessary to follow in this article all these subjects, since they are for the most part treated under separate headings, not indeed under these names—which are too comprehensive for that purpose—but under those of the more specific questions which arise under each. Thus Bionomics is treated in such articles as Evolution, Heredity, Variation, Mendelism, Reproduction, Sex, &c.; Zoo-dynamics under Medicine, Surgery, Physiology, Anatomy, Embryology, and allied articles; Plasmology under Cytology, Protoplasm, &c.; and Philosophical Zoology under numerous headings, Evolution, Biology, &c. See also Zoological Distribution, Palaeontology, Oceanography, Microtomy, &c.

It will be more appropriate here, without giving what would be a needless repetition of considerations, both historical and theoretical, which appear in other articles, to confine ourselves to two general questions, (1) the history of the various schemes of classification, or Morphography, and (2) the consideration of the main tendencies in the study of zoology since Darwin.

Systems of Classification

Morphography includes the systematic exploration and tabulation of the facts involved in the recognition of all the recent and extinct kinds of animals and their distribution in space and time, (1) The museum-makers of old days and their modern representatives the curators and describers of zoological collections, (2) early explorers and modern naturalist-travellers and writers on zoo-geography, and (3) collectors of fossils and palaeontologists are the chief varieties of zoological workers coming under this head. Gradually since the time of Hunter and Cuvier anatomical study has associated itself with the more superficial morphography until to-day no one considers a study of animal form of any value which does not include internal structure, histology and embryology in its scope.

The real dawn of zoology after the legendary period of the middle ages is connected with the name of an Englishman, Edward Wotton, born at Oxford in 1492, who practised as a physician in London and died in 1555. He published Edward Wotton. a treatise De differentiis animalium at Paris in 1552. In many respects Wetton was simply an exponent of Aristotle whose teaching, with various fanciful additions, constituted the real basis of zoological knowledge throughout the middle ages. It was Wotton’s merit that he rejected the legendary and fantastic accretions, and returned to Aristotle and the observation of nature.

The most ready means of noting the progress of zoology during the 16th, 17th and 18th centuries is to compare the classificatory conceptions of successive naturalists with those which are to be found in the works of Aristotle himself. Aristotle Aristotle’s classification.did not definitely and in tabular form propound a classification of animals, but from a study of his treatises Historia animalium, De generatione animalium, and De partibus animalium the following classification can be arrived at:—

A. Ἔναιμα, blood-holding animals ( = Vertebrata).

1. Ζωοτοκοὓντα ἐν αὺτοις, viviparous Enaema (= Mammals, including the Whale).

2. Ὄρνιθες, ( = Birds).

3. Τετράποδα ἤ ἄποδα ὠοτοκοῦντα, four-footed or legless Enaema which lay eggs (= Reptiles and Amphibia).

4. Ἰχθύες ( = Fishes).

B. Ἄναιμα, bloodless animals ( = Invertebrata).

1. Μαλάκια, soft-bodied Anaema ( = Cephalopoda).

2. Μαλακόσρακα, soft-shelled Anaema ( = Crustacea).

3. Ἔντομα, insected Anaema or Insects ( = Arthropoda, exclusive of Crustacea)

4. Ὀστρακοδέρματα, shell-bearing Anaema ( = Echini, Gastropoda and Lamellibranchia).

Wotton follows Aristotle^[4] in the division of animals into the Enaema and the Anaema, and in fact in the recognition of all the groups above given, adding only one large group to those recognized by Aristotle under the Anaema, Wotton’s modifications. namely, the group of Zoophyta, in which Wotton includes the Holothuriae, Star-Fishes, Medusae, Sea-Anemones and Sponges. Wotton divides the viviparous quadrupeds into the many-toed, double-hoofed and single-hoofed. By the introduction of a method of classification which was due to the superficial Pliny depending, not on structure, but on the medium inhabited by an animal, whether earth, air or water Wotton is led to associate Fishes and Whales as aquatic animals. But this is only a momentary lapse, for he broadly distinguishes the two kinds.

The Swiss professor, Konrad Gesner (1516–1565), is the most voluminous and instructive of these earliest writers on systematic zoology, and was so highly esteemed that his Historia animalium was republished a hundred Gesner. years after his death. His great work appeared in successive parts—e.g. Vivipara, ovipara, aves, pisces, serpentes et scorpio—and contains descriptions and illustrations of a large number of animal forms with reference to the lands inhabited by them. Gesner’s work, like that of John Johnstone (b. 1603), who was of Scottish descent and studied at St Andrews, and like that of Ulysses Aldrovandi of Bologna (b. 1522), was essentially a compilation, more or less critical, of all such records, pictures and relations concerning beasts, birds, reptiles, fishes and monsters as could be gathered together by one reading in the great libraries of Europe, travelling from city to city, and frequenting the company of those who either had themselves passed into distant lands or possessed the letters written and sometimes the specimens brought home by adventurous persons.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life, and in the latter part of the 16th century and the beginning of the 17th that careful study by “specialists” of the structure and life-history of particular groups of animals was commenced,Medical anatomists and microscopists.which, directed at first to common and familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. This minuter study had two origins, one in the researches of the medical anatomists, such as Fabricius (1537–1619), Severinus (1580–1656), Harvey (1578–1657), and Tyson (1649–1708), the other in the careful work of the entomologists and first microscopists, such as Malpighi (1628–1694), Swammerdam (1637–1680), and Hook (1635–1702). The commencement of anatomical investigations deserves notice here as influencing the general accuracy and minuteness with which zoological work was prosecuted, but it was not until a late date that their full influence was brought to bear upon systematic zoology by Georges Cuvier (1769–1832).

The most prominent name between that of Gesner and Linnaeus in the history of systematic zoology is that of John Ray (1628–1705). A chief merit of Ray is to have limited the term “species” and to have assigned to it the significance which John Ray.it bore till the Darwinian era, whereas previously it was loosely and vaguely applied. He also made considerable use of anatomical characters in his definitions of larger groups, and may thus be considered as the father of modern zoology Associated with Ray in his work, and more especially occupied with the study of the Worms and Mollusca, was Martin Lister (1638–1712), celebrated also as the author of the first geological map.

After Ray’s death the progress of anatomical knowledge, and of the discovery and illustration of new forms of animal life from distant lands, continued with increasing vigour. We note the names of Vallisnieri (1661–1730) From
Ray to Linnaeus.and Alexander Monro (1697–1767); the travellers Tournefort (1656–1708) and Shaw (1692–1751); the collectors Rumphius (1637–1706) and Hans Sloane (1660–1753); the entomologist Réaumur (1683–1757); Lhwyd (1703) and Linck (1674–1734), the students of Star-Fishes; Peyssonel (b. 1694), the investigator of Polyps and the opponent of Marsigli and Réaumur, who held them to be plants; Woodward, the palaeontologist (1665–1722)—not to speak of others of less importance.

Two years after Ray’s death Carl Linnaeus (1707–1778) was born. Unlike Jacob Theodore Klein (1685–1759), whose careful treatises on various groups of plants and animals were published during the period between Ray and Linnaeus, Linnaeus.the latter had his career marked out for him in a university, that of Upsala, where he was first professor of medicine and subsequently of natural history. His lectures formed a new departure in the academic treatment of zoology and botany, which, in direct continuity from the middle ages, had hitherto been subjected to the traditions of the medical profession and regarded as mere branches of “materia medica.” Linnaeus taught zoology and botany as branches of knowledge to be studied for their own intrinsic interest. His great work, the Systema naturae, ran through twelve editions during his lifetime (1st ed. 1735, 12th 1768). Apart from his special discoveries in the anatomy of plants and animals, and his descriptions of new species, the great merit of Linnaeus was his introduction of a method of enumeration and classification which may be said to have created systematic zoology and botany in their present form, and establishes his name for ever as the great organizer, the man who recognized a great practical want in the use of language and supplied it. Linnaeus adopted Ray’s conception of species, but he made species a practical reality by insisting that every species shall have a double Latin name—the first half to be the name of the genus common to several species, and the second half to be the specific name. Previously to Linnaeus long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus by his binomial system made it possible to write and speak with accuracy of any given species of plant or animal. He was, in fact, the Adam of zoological science. He proceeded further to introduce into his enumeration of animals and plants a series of groups, viz. genus, order, class,, which he compared to the subdivisions of an army or the subdivisions of a territory, the greater containing several of the less, as follows:—

Class.	Order.	Genus.	Species.	Variety.
Genus summum.	Genus intermedium.	Genus proximum.	Species.	Individuum.
Provincia.	Territorium.	Paroecia.	Pagus.	Domicilium.
Legio.	Cohors.	Manipulus.	Contubernium.	Miles.

Linnaeus himself recognized the purely subjective character of his larger groups; for him species were, however, objective: “there are,” he said, “just so many species as in the beginning the Infinite Being created.” It was reserved for a philosophic zoologist of the 19th century (Agassiz, Essay on Classification, 1859) to maintain that genus, order and class were also objective facts capable of precise estimation and valuation. This climax was reached at the very moment when Darwin was publishing the Origin of Species (1859), by which universal opinion has been brought to the position that species, as well as genera, orders and classes, are the subjective expressions of a vast ramifying pedigree in which the only objective existences are individuals, the apparent species as well as higher groups being marked out, not by any distributive law, but by the interaction of living matter and its physical environment, causing the persistence of some forms and the destruction of vast series of ancestral intermediate kinds.

The classification of Linnaeus (from Syst. Nat., 12th ed., 1766) should be compared with that of Aristotle. It is as follows—the complete list of Linnaean genera being here reproduced:—Classification of Linnaeus.

Class I. Mammalia.

Order 1. Primates.

Genera: Homo, Simia, Lemur, Vespertilio.

"   2. Bruta.

Genera: Elephas, Trichcrus, Bradypus, Myrmecophaga, Manis, Dasypus.

"   3. Ferae.

Genera: Phoca, Canis, Felis, Viverra, Mustela, Ursus, Didelphys, Talpa, Sorex, Erinaceus.

"   4. Glires.

Genera: Hystrix, Lepus, Castor, Mus, Sciurus, Noctilio.

"   5. Pecora.

Genera: Camelus, Moschus, Cervus, Capra, Otis, Bos.

"   6. Belluae.

Genera: Equus, Hippopotamus, Sus, Rhinoceros.

"   7. Cete.

Genera: Monodon, Balaena, Physeter, Delphinus.

Class II. Aves.

Order 1. Accipitres.

Genera: Vultur, Falco, Strix, Lanius.

"   2. Picae.

Genera: (a) Trochitus, Cerlhia, Upupa, Buphaga, Silla, Oriolus, Coracias, Gracnla, Corvus, Paradisea; (b) Ramphastos, Trogon, Psiitacus, Crotophaga, Picus. Yunx, Cuculus, Bucco; (c) Buceros, Alcedo, Merops, Todos.

"   3. Anseres.

Genera: (a) Anas, Mergus, Phaethon, Plotus; (b) Rhyncops, Diomedea, Alca, Procellaria, Pelecanus, Larus, Sterna, Colymhus.

"   4. Grallae.

Genera: (a) Phoenicopterus, Platalea, Palamedea, Myclena, Tantalus, Ardca, Recurvirostra, Scolopax, Tringa, Fuiica, Parra, Rallus, Psophia, Cancroma; (b) Hemaiopus, Charadrius, Otis, Struthio.

"   5. Gallinae.

Genera: Didus, Pavo, Meleagris, Crax, Phasianus, Telrao, Numida.

"   6. Passeres.

Genera: (a) Loxia, Fringilla, Emberiza; (b) Caprimulgus, Hirundo, Pipra; (c) Turdus, Ampelis, Tanagra, Muscicapa; (d) Parus, Molacitla, Alauda, Stiirnus, Columba.

Class III. Amphibia.

Order 1. Reptilia

Genera: Testudo, Draco, Lacerta, Rana.

"   2. Serpentes.

Genera: Crotalus, Boa, Coluber, Anguis, Amphisbaena, Caecilia.

"   3. Nantes.

Genera: Petromyzon, Raja, Squalus, Chimaera, Lophius, Acipenser, Cyclopterus, Batistes, Ostracion, Tetrodon, Diodon, Centriscus, Syngnathus, Pegasus.

Class IV. Pisces.

Order 1. Apodes.

Genera: Muraena, Gymnotus, Trichiurus, Aitarrhichas, Ammodytes, Ophidium, Stromaleus, Xiphias.

"   2. Jugulares.

Genera: Callionymus, Uranoscopus, Ttachinus, Gadus, Blentiius.

"   3. Thoracici.

Genera: Cepola, Echeneis, Coryphaena, Gobius, Cottus, Scorpaena, Zeus, Pleuronectes, Chaetodon, Sparus, Labrus, Sciaena, Perca, Gasterosteus, Scomber, Mullus, Trigla.

"   4. Abdominales.

Genera: Cobitis, Amia, Silurus, Zeuthis, Loricaria, Salmo, Fistularia, Esox, Elops, Argentina, Atherina, Mugil, Mormyrus, Exocoetus, Polynemus, Clupea, Cyprinus.

Class V. Insecta.

Order 1. Coleoptera.

Genera: (a) Scarabaeus, Lueanus, Dermesles, Hisier, Byrrhus, Gyrinus, Attelabus, Curculio,Silpha, Coccinella; (b) Bruchus, Cassida, Plinus,Chrysomela, Hispa, Meloe, Tenebrio, Lampyris, Mordella, Staphylinus; (c) Cerambyx, Leptura, Cantharis, Elater, Cicindela, Buprestis, Dytiscus, Carabus, Necydalis, Forficula.

”   2. Hemiptera.

Genera: Blatta, Mantis, Gryllus, Fulgora, Cicada, Notonecta, Nepa, Cimex, Aphis, Chermes, Coccus,Thrips.

”   3. Lepidoptera.

Genera: Papilio, Sphinx, Phalaena.

”   4. Neuroptera.

Genera: Libellula, Ephemera, Myrmeleon, Phryganea, Hemerobius, Panorpa, Raphidia.

”   5. Hymenoptera.

Genera: Cynips, Tenthredo, Sircx, Ichneumon, Sphex, Chrysis, Vespa, Apis, Formica, Mulilla.

”   6. Diptera.

Genera: Oestrus, Tipula, Musca, Tabanus, Culex, Empis, Conops, Asilus, Bombylius, Hippobosca.

”   7. Aptera.

Genera: (a) Pedibus sex; capite a thorace discreto: Lepisma, Podura, Terraes, Pediculus, Pulex.

(b) Pedibus 8–14; capite thoraceque unitis: Acarus, Phalangium, Aranea, Scorpio, Cancer, Monocidus, Oniscus.

(c) Pedibus pluribus; capite a thorace discreto: Scolopendra, Julus.

Class VI. Vermes.

Order I. Intestina.

Genera: (a) Pertusa laterali poro; Lumbricus, Sipunculus, Fasciola.

(b) Imperforata poro laterali nullo: Gordius, Ascaris, Hirtido, Myxine.

”   2. Mollusca.

Genera: (a) Ore supero; basi se affigens; Actinia, Ascidia.

(b) Ore antico; corpore pertuso laterali foraminulo: Liniax, Aplysia, Doris, Tethis.

(c) Ore antico; corpore tentaculis antice cincto: Holothuria, Terebella,

(d) Ore antico; corpore brachiato: Triton, Sepia, Clio, Lernaea, Scyllaea.

(e) Ore antico; corpore pedato: Aphrodita, Nereis.

(f) Ore infero centrali: Medusa, Asterta, Echinus.

”   3. Testacea.

Genera: (a) Multivalvia: Chiton, Lepas, Pholas.

(b) Bivalvia ( = Conchae): Mya, Solen, Tellnia, Cardium, Mactra, Donax, Venius, Spondylus, Chama, Arca, Ostrea, Anomia, Mytilus, Pinna.

(c) Univalvia spira regulari ( = Cochleae): Argonauta, Nautilus, Conns, Cypraea, Bidla, Voluta, Buccinum, Strombtus, Murex, Trochus, Turbo, Helix, Nerila, Haliotis.

(d) Univalvia absque spira regulari: Patella, Dentalium, Serpula, Teredo, Sabella.

”   4. Lithophyta.

Genera. Tubipora, Madrepora, Millepora, Cellepora.

”   5. Zoophyta.

Genera: (a) Fixata: Isis, Gorgovia, Alcyomum, Spongia, Flustra, Tubulana, Coralhna, Sertulana, Vorticella.

(b) Locomotiva: Hydra, Pennatula, Taema, Volvox, Fiiria, Chaos.

The characters of the six classes are thus given by Linnaeus:—

Cor biloculare, biauritum;	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	viviparis, Mammalibus;
⁠Sanguine calido, rubro:		oviparis, Avibus.
Cor uniloculare, uniauritum;[5]	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	pulmone arbitrario, Amphibiis;
⁠Sanguine frigido, rubro:		branchiis externis, Piscibus.
Cor uniloculare, inauritum;	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	antennatis, Insectis;
⁠Sanie frigida, albida:		tentaculatis, Vermibus.

Between Linnaeus and Cuvier there are no very great names; but under the stimulus given by the admirable method and system of Linnaeus observation and description of new forms from all parts of the world, both recent and fossil, accumulated. From
Linnaeus
to Cuvier.We can only cite the names of Charles Bonnet (1720–1793), the entomologist, who described the reproduction of Aphis; Banks and Solander, who accompanied Captain Cook on his first voyage(1768–1771); Thomas Pennant (1726–1708), the describer of the English fauna; Peter Simon Pallas (1741–1811), who specially extended the knowledge of the Linnaean Vermes, and under the patronage of the empress Catherine explored Russia and Siberia; De Geer (1720–1778), the entomologist; Lyonnet (1707–1789), the author of the monograph of the anatomy of the caterpillar of Cossus ligniperdus; Cavolini (1756–1810), the Neapolitan marine zoologist and forerunner of Della Chiaje (fl. 1828); O. F. Müller (1730–1784), the describer of fresh-water Oligochaeta, Abraham Trembley (1700–1784), the student of Hydra; and O. F. Ledermiiller (1719–1769), the inventor of the term Infusoria. The effect of the Linnaean system upon the general conceptions of zoologists was no less marked than were its results in the way of stimulating the accumulation of accurately observed details. The notion of a scala naturae, which had since the days of classical antiquity been a part of the general philosophy of nature amongst those who occupied themselves with such conceptions, now took a more definite form in the minds of skilled zoologists. The species of Linnaeus were supposed to represent a series of steps in a scale of ascending complexity, and it was thought possible thus to arrange the animal kingdom in a single series—the orders within the classes succeeding one another in regular gradation, and the classes succeeding one another m a similar rectilinear progression.

J. B. P. de Lamarck (1744–1829) represents most completely, both by his development theory (to be further mentioned below) and by his scheme of classification, the high-water mark of the popular but fallacious conception of a scala naturae. Lamarck’s classification.His classification (1801–1812) is as follows:—

Invertebrata.

1. Apathetic Animals.

Class I. Infusoria.

Orders: Nuda, Appendiculata.

Class II. Polypi.

Orders: Ciliati (Rotifera), Denudati (Hydroids), Vaginati (Anthozoa and Polyzoa), Natantes (Crinoids).

Class III. Radiaria.

Orders: Mollia (Acalephae), Echinoderma (including Actiniae).

Class IV. Tunicata.

Orders: Bothryllaria, Ascidia.

Class V. Vermes.

Orders: Molles (Tape-Worms and Flukes), Rigiduli (Nematoids), Hispiduli (Nais, &c.), Epizoariae (Lernaeans, &c.).

2. Sensitive Animals.

Class VI. Insecta (Hexapoda).

Orders: Aptera, Diptera, Hemiptera, Lepidoptera, Hymenoptera, Neuroptera, Orthoptera, Coleoptera.

Class VII. Arachnida.

Orders: Antennato-Trachealia (= Thysanura and Myriapoda), Exantennato-Trachealia, Exantennato-Branchialia.

Class VIII. Crustacea.

Orders: Heterobranchia (Branchiopoda, Isopoda, Amphipoda, Stomapoda), Homobranchia (Decapoda).

Class IX. Annelida.

Orders. Apoda, Antennata, Sedentaria.

Class X. Cirripedia.

Orders: Sessilia, Pedunculata.

Class XI. Conchifera.

Orders: Dimyaria, Monomyaria.

Class XII. Mollusca.

Orders; Pteropoda, Gasteropoda, Trachelipoda, Cephalopoda, Heteropoda.

Vertebrata.

3. Intelligent Animals.

Class XIII. Fishes. ⁠⁠⁠Class XV. Birds.

”   XIV. Reptiles. ⁠⁠⁠ ”   XVI. Mammals.

The enumeration of orders above given will enable the reader to form some conception of the progress of knowledge relating to the lower forms of life during the fifty odd years which intervened between Linnaeus and Lamarck. The number of genera recognized by Lamarck is more than ten times as great as that recorded by Linnaeus.

We have mentioned Lamarck before his great contemporary Cuvier because, in spite of his valuable philosophical doctrine of development, he was, as compared with Cuvier and estimated as a systematic zoologist, a mere enlargement and logical outcome of Linnaeus.

The distinctive merit of G. L. Cuvier (1769–1832) is that he started a new view as to the relationship of animals, which he may be said in a large measure to have demonstrated as true by his own anatomical researches. He opposed the scala naturae Cuvier.theory, and recognized four distinct and divergent branches or embranchemens, as he called them, in each of which he arranged a certain number of the Linnaean classes, or similar classes. The embranchemens were characterized each by a different type of anatomical structure. Cuvier thus laid the foundation of that branching tree-like arrangement of the classes and orders of animals now recognized as being the necessary result of attempts to represent what is practically a genealogical tree or pedigree. Apart from this, Cuvier was a keen-sighted and enthusiastic anatomist of great skill and industry. It is astonishing how many good observers it requires to dissect and draw and record over and over again the structure of an animal before an approximately correct account of it is obtained. Cuvier dissected many Molluscs and other animals which had not previously been anatomized; of others he gave more correct accounts than had been given by earlier writers. Another special distinction of Cuvier is his remarkable work in comparing extinct with recent organisms, his descriptions of the fossil Mammalia of the Paris basin, and his general application of the knowledge of recent animals to the reconstruction of extinct ones, as indicated by fragments only of their skeletons. It was in 1812 that Cuvier communicated to the Academy of Sciences of Paris his views on the classification of animals. He says:—

“Si l’on considered le règne animal d’après les principles que nous venons de poser, en se debarassant des préjuges établis sur les divisions anciennement admises, en n'ayant égard qu’ à l’organisation et à la nature des animaux, et non pas à leur grandeur, à leur utilité, au plus on moins de connaissance que nous en avons, ni à toutes les autres circonstances accessoires, on trouvera qu’il existe quatre formes principales, quatre plans généraux, si l’on peut s'exprimer ainsi, d’après lesquels tous les animaux semblent avoir été modelés et dont les divisions ultérieures, de quelque titre que les naturalistes les aient décorées, ne sont que des modifications assez légères, fondées sur le développement, on l’addition de quelques parties qui ne changent rien à l’essence du plan.”

His classification as finally elaborated in Le Règne Animal (Paris, 1829) is asCuvier’s classification. follows:—

First Branch. Animalia Vertebrata.

Class I. Mammalia.

Orders: Bimana, Quadrumana, Carnivora, Marsupialia, Rodentia, Edentata, Pachydermata, Ruminantia, Cetacea.

Class II. Birds.

Orders: Accipitres, Passeres, Scansores, Gallinae, Grallae, Palmipedes.

Class III. Reptilia.

Orders: Chelonia, Sauria, Ophidia, Batrachia.

Class IV. Fishes.

Orders:(a) Acanthopterygii, Abdominales, Subbrachii, Apodes, Lophobranchii, Plectognathi; (b) Sturiones, Selachii, Cyclostomi.

Second Branch. Animalia Mollusca.

Class I. Cephalopoda.

Class II. Pteropoda.

Class III. Gastropoda.

Orders: Pulmonata, Nudibranchia, Inferobranchia, Tectibranchia, Heteropoda, Pectinibranchia, Tubulibranchia, Scutibranchia, Cyclobranchia.

Class IV. Acephala.

Orders: Testacea, Tunicata.

Class V. Brachiopoda.

Class VI. Cirrhopoda.

Third Branch. Animalia Articulata.

Class I. Annelides.

Orders: Tubicolae, Dorsibranchiae, Abranchiae,

Class II. Crustacea.

Orders: (a) Malacostraca: Decapoda, Stomapoda, Amphipoda, Laemodipoda, Isopoda; (b) Entomostraca: Branchiopoda, Poectiopoda, Trilobitae.

Class III. Arachnides.

Orders: Pulmonariae, Tracheariae.

Class IV Insects.

Orders: Myriapoda, Thysanura, Parasita, Suctoria, Coleoptera, Orthoptera, Hemiptera, Neuroptera, Hymenopteta, Lepidoplera, Rhipiptera, Diptera.

Fourth Branch. Animalia Radiata.

Class I. Echinoderms.

Orders: Pedicellata, Apoda.

Class II Intestinal Worms.

Orders: Nematoidea, Parenchymatosa.

Class III. Acalephae.

Orders. Simplices, Hydrostaticae.

Class IV Polypi (including the Coelentera of later authorities and the Polyzoa).

Orders: Carnosi, Gelatinosi, Polypiarii.

Class V. Infusoria.

Orders: Rotifera, Homogenea (this includes the Protozoa of recent writers and some Protophyta).

The leading idea of Cuvier, his four embranchemens, was confirmed by the Russo-German naturalist Von Baer (1792–1876), who adopted Cuvier’s divisions, speaking of them as the peripheric, the longitudinal, the massive, and the vertebrate types of structure. Von Baer, however, Von Baer.has another place in the history of zoology, being the first and most striking figure in the introduction of embryology into the consideration of the relations of animals to one another.

Cuvier may be regarded as the zoologist by whom anatomy was made the one important guide to the understanding of the relations of animals. But the belief, dating from Malpighi (1670), that there is a relationship to be discovered,and not merely a haphazard The morphologists.congregation of varieties ofstructure to be classified, had previously gained ground.Cuvier was farniliar with the speculations of the “Natur-philosophen,” and with the doctrine of transmutation and filiation by which they endeavoured to account for existing animal forms. The noble aim of F. W. J. Schelling, “das ganze System der Naturlehre von dem Gesetze der Schwere bis zu den Bildungstrieben der Organismus als ein organisches Ganze darzustellen,” which has ultimately been realized through Darwin, was a general one among the scientific men of the year 1800. Lamarck accepted the development theory fully, and pushed his speculations far beyond the realm of fact. The more cautious Cuvier adopted a view of the relationships of animals which, whilst denying genetic connexion as the explanation, recognized an essential identity of structure throughout whole groups of animals. This identity was held to be due to an ultimate law of nature or the Creator’s plan. The tracing out of this identity in diversity, whether regarded as evidence of blood-relationship or as a remarkable display of skill on the part of the Creator in varying the details whilst retaining the essential, became at this period a special, pursuit, to which Goethe, the poet, who himself contributed importantly to it, gave the name “morphology.” C. F. Wolff, Goethe and Oken share the credit of having initiated these views, in regard especially to the structure of flowering plants and the Vertebrate skull. Cuvier’s doctrine of four plans of structure was essentially a morphological one, and so was the single-scale doctrine of Buffon and Lamarck, to which it was opposed. Cuvier’s morphological doctrine received its fullest development in the principle of the “correlation of parts,” which he applied to palaeontological investigation, namely, that every animal is a definite whole, and that no part can be varied without entailing correlated and law-abiding variations in other parts, so that from a fragment it should be possible, had we a full knowledge of the laws of animal structure or morphology, to reconstruct the whole. Here Cuvier was imperfectly formulating, without- ecognizing the real physical basis of the phenomena, the results of the laws of heredity, which were subsequently investigated and brought to bear on the problems of animal structure by Darwin.

Sir Richard Owen (1804–1892) may be regarded as the foremost of Cuvier’s disciples. Owen not only occupied himself with the dissection of rare animals, such as the Nautilus, Lingula, Limulus, Protopterus, Apteryx, &c., and with the description and reconstruction of extinct reptiles, birds and PearlyOwen.mammals—following the Cuvierian tradition—but gave precision and currency to the morphological doctrines which had taken their rise in the beginning of the century by the introduction of two terms, “homology” and “analogy,” which were defined so as to express two different kinds of agreement in animal structures, which, owing to the want of such “counters of thought,” had been hitherto continually confused.

Analogous structures in any two animals compared were by Owen defined as structures performing similar functions, but not necessarily derived from the modification of one and the same part in the “plan” or “archetype” according to which the two animals compared were supposed to be constructed. Homologous structures were such as, though greatly differing in appearance and detail from one another, and though performing widely different functions, yet were capable of being shown by adequate study of a series of intermediate forms to be derived from one and the same part or organ of the “plan-form” or “archetype.” It is not easy to exaggerate the service rendered by Owen to the study of zoology by the introduction of this apparently small piece of verbal mechanism; it takes place with the classificatory terms of Linnaeus. And, though the conceptions of “archetypal morphology,” to which it had reference, are now abandoned in favour of a genetic morphology, yet we should remember, in estimating the value of this and of other speculations which have given place to new views in the history of science, the words of the great reformer himself. “Erroneous observations are in the highest degree injurious to the progress of science, since they often persist for a long time. But erroneous theories, when they are supported by facts, do little harm, since every one takes a healthy pleasure in proving their falsity” (Darwin). Owen’s definition of analogous structures holds good at the present day. His homologous structures are now spoken of as “homogenetic” structures, the idea of community of representation in an archetype giving place to community of derivation from a single representative structure present in a common ancestor. Darwinian morphology has further rendered necessary the introduction of the terms “homoplasy” and “homoplastic” (E. Ray Lankester, in Ann. and Mag. Nat. Hist. 1870) to express that close agreement in form which may be attained in the course of evolutional changes by organs or parts in two animals which have been subjected to similar moulding conditions of the environment, but have not a close genetic community of origin, to account for their similarity in form and structure, although they have a certain identity in primitive quality which is accountable for the agreement of their response to similar moulding conditions.

The classification adopted by Owen in his lectures (1855) does not adequately illustrate the progress of zoological knowledge between Cuvier’s death and that date, but, such as it is, it is worth citing here.Owen’s classification.

Province: Vertebrata (Myelencephala, Owen).

Classes: Mammalia, Aves, Reptilia, Pisces.

Province: Articulata.

Classes: Arachnida, Insecta (including Sub-Classes Myriapoda, Hexapoda), Crustacea (including Sub-Classes Entomostraca, Malacostraca), Epizoa (Epizootic Crustacea), Annellata (Chaetopods and Leeches), Cirripedia.

Province: Mollusca.

Classes: Cephalopoda, Gasteropoda, Pteropoda, Lamellibranchiata, Branchiopoda, Tunicata.

Province: Radiata.

Sub-Province: Radiaria.

Classes: Echinodermata, Bryozoa, Anthozoa, Acalephae, Hydrozoa.

Sub-Province: Entozoa.

Classes: Coelelmintha, Sterelmintha.

Sub-Province: Infusoria.

Classes: Rotifera, Polygastria (the Protozoa of recent authors).

The real centre of progress of systematic zoology was no longer in France nor with the disciples of Cuvier in England, but after his death moved to Germany. The wave of morphological speculation, with its outcome of new systems and new theories of classification (see Agassiz, Essay on Classification, 1859), which were as numerous as the professors of zoological science, was necessarily succeeded in the true progress of the science by a period of minuter study in which the microscope, the discovery of embryological histories, and the all-important cell-theory came to swell the stream of exact knowledge.

The greatest of all investigators of animal structure in the 19th century was Johann Müller (1801–1858), the successor in Germany of the anatomists Rathke (1793–1860) and Meckel (1781–1833). His true greatness can only be estimated by a consideration of the fact that he was a great teacher not only of human and comparative anatomyMüller. and zoology but also of physiology, and that nearly all the most distinguished German zoologists and physiologists of the period 1850 to 1870 were his pupils and acknowledged his leadership. The most striking feature about Johann Müller’s work, apart from the comprehensiveness of his point of view, in which he added to the anatomical and morphological ideas of Cuvier a consideration of physiology, embryology and microscopic structure, was the extraordinary accuracy, facility and completeness of his recorded observations. He could do more with a single specimen of a rare animal (e.g. in his memoir on Amphioxus, Berlin, 1844) in the way of making out its complete structure than the ablest of his contemporaries or successors could do with a plethora. His power of rapid and exhaustive observation and of accurate pictorial reproduction was phenomenal. His most important memoirs, besides that just mentioned, are those on the anatomy and classification of Fishes, on the Caecilians and on the developmental history of the Echinoderms.

A name which is apt to be forgotten in the period between Cuvier and Darwin, because its possessor occupied an isolated position in England and was not borne up by any great school or university, is that of John Vaughan Thompson (1779–1847), an army surgeon, who in 1816 became district medical inspector at Cork, and then took to the J. V. Thompson. study of marine Invertebrata by the aid of the microscope. Thompson made three great discoveries, which seem to have fallen in his way in the most natural and simple manner, but must be regarded really as the outcome of extraordinary genius. He showed (1830) that the organisms like Flustra are not hydroid Polyps, but of a more complex structure resembling Molluscs, and he gave them the name Polyzoa. He discovered (1823) the Pentacrinus europaeus, and showed that it was the larval form of the Feather-Star Antedon (Comatula). He upset (1830) Cuvier’s retention of the Cirripedes among Mollusca, and his subsequent treatment of them as an isolated class, by showing that they begin life as free-swimming Crustacea identical with the young forms of other Crustacea. Vaughan Thompson is a type of the marine zoologists, such as Dalyell, Michael Sars, P. J. Van Beneden, Claparède, and Allman, who during the 19th century approached the study of the lower marine organisms in the same spirit as that in which Trembley and Schaffer in the 18th century, and Swammerdam in the 17th, gave themselves to the study of the minute fresh-water forms of animal life.

It is impossible to enumerate or to give due consideration to all the names in the army of anatomical and embryological students of the middle third of the 19th century whose labours bore fruit in the modification of zoological theories and in the building up of a true classification of animals. Their results are best summed up in the three schemes of classification which follow below— those of Rudolph Leuckart (1823–1896), Henri Milne-Edwards (1800–1884), and T. H. Huxley (1825–1895), all of whom individually contributed very greatly by their special discoveries and researches to the increase of exact knowledge.

Contemporaneous with these were various schemes of classification which were based, not on a consideration of the entire structure of each animal, but on the variations of a single organ, or on the really non-significant fact of tact the structure of the egg. All such single-fact systems systems have proved to be departures from the Single fact systems of classification.true line of growth of the zoological system which was shaping itself year by year—unknown to those who so shaped it—as a genealogical tree. They were attempts to arrive at a true knowledge of the relationships of animals by “royal roads”; their followers were landed in barren wastes.

R. Leuckart’s classification (Die Morphologie und die Verwandtschaftsverhältnisse der wirbellosen Thdere, Brunswick, 1848) is as follows:—Leuckart’s classification.

Type 1. Coelenterata.

Class I. Polypi.

Orders: Anthozoa and Cylicozoa,

”  II. Acalephae.

Orders: Discophorae and Ctenopnorae.

Type 2. Echinodermata.

Class I. Pelmatozoa.

Orders: Cystidea and Crinoidea.

”  II. Actinozoa.

Orders: Echinida and Asterida.

”  III. Scytodermata.

Orders: Holothuriae and Sipunculida.

Type 3. Vermes.

Class I. Anenteraeti.

Orders: Cestodes and Acanthocephali.

”  II. Apodes.

Orders: Nemertini, Tubellarii, Trematodes and Hirudinei.

”  III. Ciliati.

Orders: Bryozoa and Rotifera.

”  IV. Annelides.

Orders: Nematodes Lumbricini and Branchiati.

Type 4. Arthropoda.

Class I. Crustacea.

Orders: Entomostraca and Malacostraca.

”  II. Insecta.

Orders: Myriapoda, Arachnida (Acera, Latr.), and Hexapoda.

Type 5. Mollusca.

Class I. Tunicata.

Orders: Ascidiae and Salpae.

”  II. Acephala.

Orders: Lamellibranchiata and Brachiopoda.

”  III. Gasteropoda.

Orders: Heterobranchia, Dermatobranchia, Heteropoda, Ctenobranchia, Pulmonata, and Cyclobranchia.

”  IV. Cephalopoda.

Type 6. Vertebrata. (Not specially dealt with.)

Milne-Edwards’s classification.The classification given by Henri Milne-Edwards (Cours Élementaire d’Histoire Naturelle, Paris, 1855) is as follows:—

Branch I. Osteozoaria or Vertebrata.

Sub-Branch 1. Allantoidians.

Class I. Mammalia.

Orders: (a) Monodelphia: Bimana, Quadrumana, Cheiroptera, Insectivora, Rodentia, Edentata, Carnivora, Amphibia, Pachydermata, Ruminantia, Cetacea; (b) Didelphia: Marsupialia, Monotremata.

”  II. Birds.

Orders: Rapaces, Passeres, Scansores, Gallinae, Grallae, Palmipedes.

”  III. Reptiles.

Orders: Chelonia, Sauria, Ophidia.

Sub-Branch 2. Anallantoidians.

Class I. Batrachians.

Orders: Anura, Urodela, Perennibranchia, Caeciliae.

”  II. Fishes.

Section 1. Ossei.

Orders: Acanthopterygii, Abdominales, Subbrachii, Apodes, Lophobranchii, Plectognathi.

Section 2. Chondropterygii.

Orders: Sturiones, Selachii, Cyclostomi.

Branch II. Entomozoa or Annelata.

Sub-Branch 1. Arthropoda.

Class I. Insecta.

Orders: Coleoptera, Orthoptera, Neuroptera, Hymenoptera, Lepidoptera, Hemiptera, Diptera, Rhipiptera, Anopleura, Thysanura.

”  II. Myriapoda.

Orders: Chilognatha and Chilopoda.

”  III. Arachnids.

Orders: Pulmonaria and Trachearia,

”   IV. Crustacea.

Section I. Podophthalmia.

Orders: Decapoda and Stomopoda.

Section 2. Edriophthalmi.

Orders: Amphipoda, Loemodipoda and Isopoda.

Section 3. Branchiopoda.

Orders: Ostracoda, Phyllopoda and Trilobilae.

Section 4. Entomostraca.

Orders: Copepoda, Cladocera, Siphononostoma, Lernaeida, Cirripedia,

Section 5. Xiphosura.

(The orders of the classes which follow are not given in the work quoted.)

Sub-Branch 2. Vermes.

Class I. Annelids.

”  II. Helminths.

”  III. Turbellaria.

Class IV. Cestoidea.

”  V. Rotatoria.

Branch III. Malacozoaria or Mollusca.

Sub-Branch 1. Mollusca proper,

Class I. Cephalopoda.

”  II. Pteropoda.

Class III. Gasteropoda.

”  IV. Acephala.

Sub-Branch 2. Molluscoidea.

Class I. Tunicata.

Class II. Bryozoa.

Branch IV. Zoophytes.

Sub-Branch 1. Radiaria.

Class I. Echinoderms.

”   II. Acalephus.

Class III. Corallaria or Polypi.

Sub-Branch 2. Sarcodaria.

Class I. Infusoria.

Class II. Spongiaria.

In England T. H. Huxley adopted in his lectures (1869) a classification which was in many respects similar to both of the foregoing, but embodied improvements of his own. It is as follows:—Huxley’s classification.

Sub-Kingdom I. Protozoa.

Classes: Rhizopoda, Gregarinida, Radiolaria, Spongida.

Sub-Kingdom II. Infusoria.

Sub-Kingdom III. Coelenterata.

Classes: Hydrozoa, Actinozoa.

Sub-Kingdom IV. Annuloida.

Classes: Scolecida, Echinodermata.

Sub-Kingdom V. Annulosa.

Classes: Crustacea, Arachnida, Myriapoda. Insecta. Chaetognatha, Annelida.

Sub-Kingdom VI. Molluscoida.

Classes: Polyzoa, Brachiopoda, Tunicata.

Sub-Kingdom VII. Mollusca.

Classes: Lamellibranchiata, Branchiogastropoda. Pulmogastropoda, Pteropoda, Cephalopoda,

Sub-Kingdom VIII. Vertebrata.

Classes: Pisces, Amphibia, Reptilia, Aves, Mammalia.

We now arrive at the period when the doctrine of organic evolution was established by Darwin, and when naturalists, being convinced by him as they had not been by the transmutationists of fifty years’ earlier date, were compelled to take an entirely new view of the significance of all attempts at framing a “natural” classification.

Many zoologists—prominent among them in Great Britain being Huxley—had been repelled by the airy fancies and assumptions of the “philosophical” morphologists. The efforts of the best minds in zoology had been directed for thirty years or more to ascertaining with increased accuracy and Classification based on structure.minuteness the microscopic and gross, of all possible forms of animals, and not only of the adult structure but of the steps of development of that structure in the growth of each kind of organism from the egg to maturity. Putting aside fantastic theories, these observers endeavoured to give in their classifications a strictly objective representation of the facts of animal structure and of the structural relationships of animals to one another capable of demonstration. The groups within groups adopted for this purpose were necessarily wanting in symmetry: the whole system presented a strangely irregular character. From time to time efforts were made by those who believed that the Creator must have followed a symmetrical system in his production of animals to force one or other artificial, neatly balanced scheme of classification, upon the zoological world. .The last of these was that of Louis Agassiz (1807–1873), who, whilst surveying all previousAgassiz. classifications, propounded a scheme of his own (Essay on Classification, 1859), in which, as well as in the criticisms he applies to other systems, the leading notion is that sub-kingdoms, classes, orders and families have a real existence, and that it is possible to ascertain and distinguish characters which are of class value, others which are only of ordinal value, and so on, so that the classes of one sub-kingdom should on paper, and in nature actually do, correspond in relative value to those of another sub-kingdom, and the orders of any one class similarly should be so taken as to be of equal value with those of another class, and have been actually so created.

The whole position was changed by the acquiescence, which became universal, in the doctrine of Darwin. That doctrine took some few years to produce its effect, but it became evident at once to those who accepted Darwinism that the natural classification Influence of Darwinian doctrine on taxonomy.of animals, after which collectors and anatomists, morphologists, philosophers and embryologists had been so long striving, was nothing more nor less than a genealogical tree, with breaks and gaps of various extent in its record. The facts of the relationships of animals to one another, which had been treated as the outcome of an inscrutable law by most zoologists and glibly explained by the transcendental morphologists, were amongst the most powerful arguments in support of Darwin’s theory, since they, together with all other vital phenomena, received a sufficient explanation through it. It is to be noted that, whilst the zoological system took the form of a genealogical tree, with main stem and numerous diverging branches, the actual form of that tree, its limitation to a certain number of branches corresponding to a limited number of divergences in structure, came to be regarded as the necessary consequence of the operation of the physico-chemical laws of the universe, and it was recognized that the ultimate explanation of that limitation is to be found only in the constitution of matter itself.

The first naturalist to put into practical form the consequences of the new theory, in so far as it affected zoological classification, was Ernst Haeckel of Jena (b. 1834), who in 1866, seven years after the publication of Darwin’s Haeckel.Origin of Species, published his suggestive Generelle Morphologie. Haeckel introduced into classification a number of terms intended to indicate the branchings of a genealogical tree. The whole “system” or scheme of classification was termed a genealogical tree (Stammbaum); the main branches were termed “phyla,” their branching’s “sub-phyla”; the great branches of the sub-phyla were termed “cladi,” and the “cladi” divided into “classes,” these into sub-classes, these into legions, legions into orders, orders into sub-orders, suborders into tribes, tribes into families, families into genera, genera into species. Additional branchings could be indicated by similar terms where necessary. There was no attempt in Haeckel’s use of these terms to make them exactly or more than approximately equal in significance; such attempts were clearly futile and unimportant where the purpose was the exhibition of lines of descent, and where no natural equality of groups was to be expected ex hypothesi. Haeckel’s classification of 1866 was only a first attempt. In the edition of the Natürliche Schöpfungsgeschichte published in 1868 he made a great advance in his genealogical classification, since he now introduced the results of the extraordinary activity in the study of embryology which followed on the publication of the Origin of Species.

The pre-Darwinian systematists since the time of Von Baer had attached very great importance to embryological facts, holding that the stages in an animal’s development were often more significant of its true affinities than its adult structure Von Baer had gained unanimous support for his dictum, “Die EntwickelungsgescTiichte ist der wahre Lichttrager für Untersuchungen iiber organische Körper.” Thus J. Müller’s studies on the larval forms of Echinoderms and the discoveries of Vaughan Thompson were appreciated. But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell. Kölliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier. Kovalevsky’s work appeared between the dates of the Generelle Morpholgie and the Schöpfungsgcschihite. Haeckel himself, with his pupil Miklucho-Maclay, had in the meantime made studies on the growth from the egg of Sponges— studies which resulted in the complete separation of the unicellular or equicellular Protozoa from the Sponges, hitherto confounded with them. It is this introduction of the consideration of cell-structure and cell-development which, subsequently to the establishment of Darwinism, has most profoundly modified the views of systematists, and led in conjunction with the genealogical doctrine to the greatest activity in research—an activity which culminated in the work (1873–1882) of F. M. Balfour, and produced the profoundest modifications in classification.

Haeckel’s 1868 arrangement.Haeckel’s second pedigree is as follows:—

Phyla	Clades		Classes
Protozoa.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Ovularia.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Archezoa.
				Gregarinae.
				Infusoria.
		Blastularia.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Planeada.
				Gastreada.
Zoophyta.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Spongiae.		Porifera.
		Acalephae.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Coralla.
				Hydromedusae.
				Ctenophora.
Vermes.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Acoelomi.		Platyhelminthes.
		Coelomati.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Nemathelminthes.
				Bryozoa.
				Tunicata.
				Rhynchocoela.
				Gephyraea.
				Rotatoria.
				Annelida.
Mollusca.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Acephala.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Spirobranchia.
				Lamellibranchia.
		Eucephala.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Cochlides.
				Cephalopoda.
Echinoderma.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Colobrachia.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Asterida.
				Crinoida.
		Lipobrachia.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Echinida.
				Holothuriae.
Arthropoda.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Carides.		Crustacea.
		Tracheata.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Arachnida.
				Myriapoda.
				Insecta.
Vertebrata.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Acrania.		Leptocardia.
		Monorrhina.		Cyclostoma.
		Anamnia.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Pisces.
				Dipneusta.
				Halisauria.
				Amphibia.
		Amniota.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Reptilia.
				Aves.
				Mammalia.

In representing pictorially the groups of the animal kingdom as the branches of a tree, it becomes obvious that a distinction may be drawn, not merely between the individual main branches, but further as to the Dendriform distribution of animal kingdom.level at which they are given off from the main stem, so that one branch or set of branches may be marked off as belonging to an earlier or lower level than another set of branches; and the same plan may be adopted with regard to the clades, classes and smaller branches. The term “grade” was introduced by Ray Lankester (“Notes, on Embryology and Classification,” in Quart. Journ. Micr. Sci. 1877), to indicate this giving off of branches at a higher or lower, i.e. a later or earlier, level of a main stem.^[6] The mechanism for the statement of the genealogical relationships of the groups of the animal kingdom was thus completed. Renewed study of every group was the result of the acceptance of the genealogical idea and of the recognition of the importance of cellular embryology. On the one hand, the true method of arriving at a knowledge of the genealogical tree was recognized as lying chiefly in attacking the problem of the genealogical relationships of the smallest twigs of the tree, and proceeding from them to the larger branches. Special studies of small families or orders of animals with this object in view were taken in hand by many zoologists. On the other hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kovalevsky’s work led to a generalization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg-cell by “cell division” gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron. Of these latter, two grades were further distinguished by Lankester—those which remain possessed of a single archenteric cavity and of two primary cell-layers (the Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or body-cavity and the metenteron or gut (Coelomata or Triploblastica). To the primitive two-cell-layered form, the hypothetical ancestor of all Metazoa or Enterozoa, Haeckel gave the name Gastraea; the embryonic form which represents in the individual growth from the egg this ancestral condition he called a “gastrula.” The term “diblastula” was subsequently adopted in England for the gastrula of Haeckel. The tracing of the exact mode of development, cell by cell, of the diblastula, the coelom, and the various tissues of examples of all classes of animals was in later years pursued with immense activity and increasing instrumental facilities.

Two names in connexion with post-Darwinian taxonomy and the ideas connected with it require brief mention here. Fritz Müller, by his studies on Crustacea (Für Darwin, 1864), showed the way in which Fritz Müller’s recapitu-
lation.genealogical theory may be applied to the minute study of a limited group. He is also responsible for the formulation of an important principle, called by Haeckel “the biogenetic fundamental law,” viz. that an animal in its growth from the egg to the adult condition tends to pass through a series of stages which are recapitulative of the stages through which its ancestry has passed in the historical development of the species from a primitive form; or, more shortly, that the development of the individual (ontogeny) is an epitome of the development of the race (phylogeny). Pre-Darwinian zoologists had been aware of the class of facts thus interpreted by Fritz Müller, but the authoritative view on the subject had been that there is a parallelism between (a) the series of forms which occur in individual development, (b) the series of existing forms from lower to higher, and (c) the series of forms which succeed one another in the strata of the earth’s crust, whilst an explanation of this parallelism was either not attempted, or was illusively offered in the shape of a doctrine of harmony of plan in creation. It was the application of Fritz Müller’s law of recapitulation which gave the chief stimulus to embryological investigations between 1865 and 1890; and, though it is now recognized that “recapitulation” is vastly and bewilderingly modified by special adaptations in every case, yet the principle has served, and still serves, as a guide of great value.

Another important factor in the present condition of zoological knowledge as represented by classification is the doctrine of degeneration propounded by Anton Dohrn. Lamarck believed in a single progressive series of forms whilst Cuvier introduced conception of branches. Dohrn’s doctrine
of degen-
eration.The first post-Darwinian systematists naturally and without reflexion accepted the idea that existing simpler forms represent stages gradual progress of development—are in fact survivors from past ages which have retained the exact grade of development which their ancestors had reached in past ages. The assumption made was that (with the rare exception of parasites) all the change of structure through which the successive generations of animals have passed has been one of progressive elaboration. It is Dohrn’s merit to have pointed out^[7] that this assumption is not warranted, and that degeneration or progressive simplification of structure may have, and in many lines certainly has, taken place, as well as progressive elaboration and in other cases continuous maintenance of the status quo. The introduction of this conception necessarily has had a most important effect in the attempt to unravel the genealogical affinities of animals. It renders the task a more complicated One; at the same time it removes some serious difficulties and throws a flood of light on every group of the animal kingdom.

One result of the introduction of the new conceptions dating from Darwin was a healthy reaction from that attitude of mind which led to the regarding of the classes and orders recognized by authoritative zoologists as sacred institutions which were beyond the criticism of ordinary men. That state of mind was due to the fact that the groupings so recognized did not profess to be simply the result of scientific reasoning, but were necessarily regarded as the expressions of the “insight” of some more or less gifted persons into a plan or system which had been arbitrarily chosen by the Creator. Consequently there was a tinge of theological dogmatism about the whole matter. To deny the Linnaean, or later the Cuvierian, classes was very much like denying the Mosaic cosmogony. But systematic zoology is now entirely free from any such prejudices, and the Linnaean taint which is apparent even in Haeckel and Gegenbaur may be considered as finally expunged.

Sub-Grade B. CŒLOMATA.

Sub-Grade A. CŒLENTERA.
Grade 2. ENTEROZOA.

Grade 1. PROTOZOA.

A genealogical tree of animal kingdom (Lankester, 1884).

There are, and probably always will be, differences of opinion as to the exact way in which the various kinds of animals may be divided into groups and those groups arranged in such an order as will best exhibit Lankester’s system.their probable genetic relationships. The main divisions which, writing in 1910, the present writer prefers, are those adopted in his Treatise on Zoology (Part II. ch. ii.) except that Phylum 17, Diplochorda (a name doubtfully applicable to Phoronis) is replaced by Podaxonia, a term employed by Lankester in the 9th edition of this-encyclopedia and now used to include a number of groups of doubtful but possible affinity. The terms used for indicating groups are “Phylum” for the large diverging branches of the genealogical tree as introduced by Haeckel, each Phylum bears secondary branches which are termed “classes,” classes again branch or divide into orders, orders into families, families into genera, genera into species. The general purpose is to give something like an equivalence of importance to divisions or branches indicated by the same term, but it is not intended to imply that every phylum has the same range and distinctive character as every other, nor to make such a proposition about classes, orders, families and genera. Where a further subdivision is desirable without descending to the next lower term of grouping, the prefix "sub" is made use of, so that a class may be divided first of all into subclasses each of which is divided into orders, and an order into sub-orders each of which bears a group of families. The term "grade" is also made use of for the purpose of indicating the conclusion that certain branches on a larger or smaller stem of the genealogical tree have been given off at an earlier period in the history of the evolution of the stem in question than have others marked off as forming a higher grade. Thus, to begin with, the animal pedigree is divided into two very distinct grades, the Protozoa and the Metazoa. The Metazoa form two main branches; one, Parazoa, is but a small unproductive stock comprising only the Phylum Porifera or Sponges; the other, the great stem of the animal series Enterozoa, gives rise to a large number of diverging Phyla which it is necessary to assign to two levels or grades — a lower, Enterocoela (often called Coelentera), and a higher, Coelomocoeta (often called Coelomata). These relations are exhibited by the two following diagrams.

Diagram showing the primary grades and branches of the Animal Pedigree.

Diagram to show the division of the great branch Enterozoa into two grades and the Phyla given off therefrom.

The Phylum Vertebrata in the above scheme branches into the sub-phyla Hemichorda, Urochorda, Cephalochorda and Craniata. The Phylum Appendiculata similarly branches into sub-phyla, viz. the Rotifera, the Chaetopoda and the Arthropoda. Certain additional small groups should probably be recognized as independent lines of descent or phyla, but their relationships are obscure—they are the Mesozoa, the Polyzoa, the Acanthocephala and the Gastrotricha.

We may now enumerate these various large groups in tabular form.

BIONTA—Phyta, Animalia.

Grade A. Protozoa (various groups included).

Grade B. Metazoa.

Branch a. Parazoa.

Phylum 1. Porifera.

Branch b. Enterozoa.

Grade 1. Enterocoela.

Phylum 2. Hydromedusae.

⁠3. Scyphomedusae.

⁠4. Anthozoa.

⁠5. Ctenophora.

Grade 2. Coelomocoela.

Phylum 6. Platyelmia.

⁠7. Nematoidea.

⁠8. Chaetognatha

⁠9. Nemertina.

⁠10. Mollusca.

⁠11. Appendiculata.

Sub-phyla: Rotifera, Chaetopoda, Arthropoda.

⁠12. Echinoderma.

⁠13. Vertebrata.

Sub-phyla: Hemichorda, Urochorda, Cephalochorda, Craniata.

⁠14. Mesozoa.

⁠15. Polyzoa.

⁠16. Acanthocephala.

⁠17. Podaxonia.

⁠18. Gastrotricha.

Classification adopted in the present work.A statement may now be given of the classes and orders in each group, as recognized by the writers of the various special zoological articles in the Eleventh Edition of the Encyclopaedia Britannica. These subdivisions of the larger groups are not necessarily those theoretically approved by the present writer, but they have the valuable sanction of the individual experts who have given special attention to different portions of the vast field represented by the animal kingdom.^[8]

Grade A. Protozoa (q.v.).

Phylum 1. Sarcodina (q.v.).

Class 1. Proteomyxa (q.v.)

Class 2. Rhizopoda (q.v.).

Orders: Lobosa, Filosa.

Class 3. Heliozoa (q.v.).

Class 4. Foraminifera (q.v.).

Orders: Nuda, Allogromidiaceae, Astrorhizidiaceae, Lilitolidaceae, Miliolidaceae, Textididaridaceae, Cheilostomellaceae, Lagenidaceae, Globigerinidaceae, Rotalidaceae, Nummutidiaceae.

Insertae sedis. Xenophyophoridae (see Foraminifera).

Class 5. Radiolaria.

Orders: Spumellaria ( = Peripylaea), Acantharia ( = Actipylaea), Nasselaria ( = Monopylaea), Phaeodaria ( = Tripylaea).

Class 6. Labyrinthulidea (q.v.).

No Orders.

Class 7. Myxomycetes.

No Orders.

Phylum 2. Mastigophora (q.v.).

Class 1. Flagellata (q.v.).

Sub-class A. Rhizoflagellata.

Orders: Holomastigaceae, Rhizomastigaceae.

Sub-class B. Euflagellata.

Orders: Protomastigaceae, Chrysomonadaceae, Cryptomonadaceae, Chloromotiadaceae, Euglenaceae, Volvocaceae.

Class 2. Dinoflagellata.

Orders: Gymnodiniaceae, Prorocentraceae, Peridiniaceae.

Class 3. Cystoflagellata.

No Orders.

Phylum 3. Sporozoa (q.v.).

Class 1. Endospora (q.v.).

Orders: Myxosporidia, Actinomyxidia, Sarcosporidia, Haplosporidia.

Class 2. Ectospora (q.v.).

Orders: Gregarina (see Gregarines), Coccidia (q.v.), Haemosporidia (q.v.).

Phylum 4. Infusoria (q.v.).

Class 1. Ciliata.

Orders; Gymonostomaceae, Trichostomata, Aspirotrichaceae, Spirotricha, Heterotrichaceae, Oligotrichaceae, Hypoltichaceae, Peritrichaceae.

Class 2. Suctoria.

No orders.

Grade B. Metazoa.

Branch a. Parazoa.

Phylum 1. Porifera (see Sponges).

Sub-phylum 1. Calcarea.

Class. Calcarea.

Orders: Homocoela, Heterocoela.

Sub-phylum 2. Non-Calcarea.

Class 1. Myxospongida.

Order: Myxospongida.

Class 2. Triaxonida ( = Hexactinellida).

Orders: Amphidiscophora, Hexasterophora.

Class 3. Tetraxonida.

Sub-Class1. Tetractinellida.

Orders: Homosclerophora, Astrophora, Sigmatophora.

Sub-class 2. Lithistida.

No Orders.

Sub-class 3. Monaxonellida.

Orders: Astromonaxonellida, Sigmatomonaxonellida.

Class 4. Euceratosa.

Order: Euceratosa.

Branch b. Enterozoa.

Grade 1. Enterocoela (see Coelentera).

Phylum 2. Hydromedusae or Hydrozoa (q.v.).

Class. Hydromedusae, (q.v.).

Orders: Eleutheroblastea, Hydroidae seu Leptolinae (Sub-orders: Anthomedusae, Leptomedusae), Hydrocorallinae, Graptolitoidea Trachylinae (Sub-orders: Trachomedusae, Narcomedusae), Siphonophora.

Phylum 3. Scyphomedusae (q.v.).

Class. Scyphomedusae.

Orders: Cubomedusae, Stauromedusae, Coronata, Discophora.

Phylum 4. Anthozoa (q.v.).

Class. Anthozoa.

Sub-class 1. Alcyonaria.

Orders: Stolonifera, Alcyonacea, Pseudaxonia, Axifera, Stelechotokea, Coenothecalia.

Sub-class 2. Zoantharia.

Orders: Zoanthidea, Cereanthidea, Antipathidea, Actiniidea (Sub-orders: Malacactiniae and Scleractiniae or Madreporia).

Phylum 5. Ctenophora.

Class. Ctenophora.

Sub-class 1. Tentaculata.

Orders: Cydippidea, Lobata, Cestoidea.

Sub-class 2. Nuda.

No Orders.

Grade 2. Coelomocoela.

Phylum 6. Platyelmia (q.v.).

Class 1. Planaria (see Planarians).

Order: Turbellaria.

Class 2. Temnocephaloidea (see appendix to Planarians).

No Orders.

Class 3. Trematoda (see Trematodes).

Orders: Heterocotylea, Aspidocotylea, Malacocotylea.

Class 4. Cestoda (see Tapeworms).

Orders: Monozoa, Merozoa.

Phylum 7. Nematoidea.

Class 1. Nematoda (see Nematode).

No Orders.

Class 2. Chaetosomidae (see Chaetosomatida).

No Orders.

Class 3. Desmoscolecida (q.v.).

No Orders.

Class 4. Nematomorpha (q.v.).

No Orders.

Phylum 8. Chaetognatha (q.v.).

No Orders.

Phylum 9. Nemertina.

Class. Nemertina (q.v.).

Orders: Protonemertini, Mesonemertini, Metanemertini, Heteronemertini.

Phylum 10. Mollusca (q.v.).

Grade A. Isopleura.

Class 1. Amphineura (see Chiton).

Orders: Polyplacophora, Aplacophora.

Grade B. Prorhipidoglossomorpha.

Class 2. Gastropoda (q.v.).

Sub-class 1. Streptoneura.

Orders: Aspidobranchia, Pectinibranchia.

Sub-class 2. Euthyneura.

Orders: Opisthobranchia, Pulmonata.

Class 3. Scaphopoda (q.v.).

No Orders.

Class 4. Lamellibranchia (q.v.).

Orders: Protobranchia, Filibranchia, Eulamellibranchia, Septibranchia.

Grade C. Siphonopoda.

Class 5. Cephalopoda (q.v.).

Orders: Tetrabranchia, Dibranchia.

Phylum 11. Appendiculata.

Sub-phylum 1. Rotifera (q.v.).

Class. Rotifera.

Orders: Aspianchnaceae, Melicerlaceae, Trochosphaeraceae, Ploimoidaceae, Bdelloidaceae, Floscularaceae, Ploima, Seisonaceae.

Sub-phylum 2. Chaetopoda (q.v.).

Class 1. Polychaeta.

Orders: Nereidiformia, Cryptocephala, Capitelliformia, Terebeliiformia, Spioniformia, Scoleciformia.

Class 2. Oligochaeta.

Orders: Aphaneura, Limicolae, Moniligasires, Terricolae.

Class 3. Hirudinae (see Leech).

Orders: Rhynchobdellidae, Gnathobdellidae, Acanthobdellidae.

Class 4. Myzostomida (q.v.).

No Orders.

Class 5. Saccocirrida.

No Orders.

Class 6. Haplodrili (q.v.).

No Orders.

Class 7. Echiuroidea (q.v.).

No Orders.

Sub-phylum 3. Arthropoda (q.v.).

Grade 1. Ceratophora.

Class 1. Peripatidea (see Peripatus).

No Orders.

Class 2. Chilopoda (see Centipede).

Sub-class 1. Pleurostigma.

Orders: Geophilomorpha, Scolopendromorpha, Craterostigmomorpha, Lithobiomorpha.

Sub-class 2. Notostigma.

Order: Scutigeromorpha.

Class 3. Diplopoda (see Millipede).

Sub-class 1. Pselaphognatha.

Order: Penicillata.

Sub-class 2. Chilognatha.

Orders: Oniscomorpha, Limacomorpha, Colobognatha, Ascospermophora, Proterospermophora, Merochaeta, Opisthospermophora.

Class 3. Pauropoda (see Millipede).

No Orders.

Class 4. Symphyla (see Millipede).

No Orders.

Class 5. Hexapoda (q.v.).

Sub-class 1. Apterygota.

Order: Aptera.

Sub-class 2. Exopterygota.

Orders: Dermaptera, Orthoptera, Plecoptera, Isoptera, Corrodentia, Ephemoptera, Odonata, Thysanoptera, Hemiptera, Anoplura.

Sub-class 3. Endopterygota.

Orders: Neuroptera, Coleoptera, Mecaptera, Trichoptera, Lepidoptera, Diptera, Siphonaptera, Hymenoptera.

Grade 2. Acerata.

Class 1. Crustacea (q.v.).

Sub-class 1. Entomostraca (q.v.).

Orders: Branchiopoda (Sub-orders: Phyllopoda, Cladocera, Branchiura), Ostracoda, Copepoda.

Sub-class 2. Thyrostraca (q.v.) = (Cirripedia).

No Orders.

Sub-class 3. Leptostraca.

No Orders.

Sub-class 4. Malacostraca (q.v.).

Orders: Decapoda (Sub-orders: Brachyura, Macrura), Schizopoda (including Anaspides), Stomatopoda, Sympoda (Cumacea), Isopoda (including Tanaidacea), Amphipoda.

Class 2. Arachnida (q.v.).

Grade A. Trilobitae (see Trilobite).

(Orders not determined.)

Grade B. Nomomeristica.

Sub-class 1. Pantopoda.

Orders: Nymphonomorpha, Ascorhynchomorpha, Pycnogonomorpha.

Sub-class 2. Eu-Arachna.

Grade a. Delobrancha (or Hydropneusta).

Orders: Xiphosura, Gigantostraca.

Grade b. Embolobranchia (or Aeropneusta).

Section. Pectinifera.

Order: Scorpionidea.

Section. Epectinata.

Orders: Pedipalpi, Araneae, Palpigradi, Solifugae, Pseudoscorpiones, Podogona, Opiliones, Rnynchostomi (Acari).

Class 3. Tardigrada (q.v.).

No Orders.

Class 4. Linguatalina (see Pentastomida).

No Orders.

Phylum 12. Echinoderma (see Echinoderms).

Branch A. Pelmatozoa.

Class 1. Cystidea.

Orders: Amphoridea, Carpoidea, Rhombifera, Aporita, Diploporita.

Class 2. Blastoidea.

Divisions: Protoblastoidea, Eublastoidea.

No Orders.

Class 3. Crinoidea.

Orders: Monocyclica Inadunata, Adunata, Monocyclica Camerata, Dicydica Inadunata, Flexibilia, Dicyclica Camerata.

Class 4. Edrioasteroidea.

No Orders.

Branch B. Eleutherozoa.

Class I. Holothuroidea.

Orders: Aspidochirota, Dendrochirota.

Class 2. Stelliformia.

Sub-class I. Asterida.

Orders: Phanerozonia, Cryptozonia.

Sub-class 2. Ophiurida.

Orders: Streptophiurae, Zygophiurae, Cladophiurae.

Class 3. Echinoidea.

Orders: Bothriocidaroida, Melonitoida, Gystocidaroida, Cidaroida, Diademoida, Holectypoida, Spatangoida, Clypeastroida.

Phylum 13. Vertebrata (q.v.).

Sub-phylum a. Hemichorda (q.v.).

Class. Enteropneusta (see Balanoglossus).

No Orders.

Sub-phylum b. Urochorda.

Class. Tunicata (q.v.).

Orders: Larvacea, Thaliacea (Sub-orders: Cyclomyaria, Hemimyaria), Ascidiacea (Sub-orders: Ascidiae Simplices, Ascidiae Compositae, Ascidiae Luciae).

Sub-phylum c. Cephalochorda (see Amphioxus).

Class. Cephalochorda.

No Orders.

Sub-phylum d. Craniata.^[9]

Class 1. Pisces (see Ichthyology).

Sub-class 1. Cyclostomata (q.v.).

Orders: Myxinoides (or Hyperotreti), Petromyzontes (or Hyperoartii).

Sub-class 2. Selachia or Elasmobranchii (see Selachians).

Orders: Pleuropterygii, Acanthodii, Ichthyotomi, Plagiostomi, Holocephali.

Sub-class 3. Teleostoma.

Orders: Ganoidea, Crossopterygii, Dipneusti, Teleostei.

Class 2. Batrachia (q.v.).

Orders: Stegocephalia, Apoda (or Peromela), Caudata (or Urodela), Ecaudata (or Anura).

Class 3. Reptilia (see Reptiles).

Orders: Anomodontia, Chelonia, Samopterygia, Ichthyopterygia, Rhyncocephalia, Dinosauria, Crocodilia, Ornithosauria, Squamata.

Class 4. Aves (see Bird and Ornithology).

Sub-class 1. Archaeornithes.

No Orders.

Sub-class 2. Neornithes.

Division 1. Ratitae.

Orders: Struthiones, Rheae, Casuariae, Apteryges, Dinornithes, Aepyomithes.

Division 2. Odontolcae.

No Orders.

Division 3. Carinatae.

Orders: Ichthyornes, Colymbiformes, Sphenisciformes, Procellariiformes, Ciconiiformes, (Sub-orders: Steganopodes, Ardeae, Ciconiae, Phoenicopteri). Anserifermes (Sub-orders: Palamedeae, Anseres), Falconiformes (Sub-orders: Cathartae, Accipitres), Tinamiformes, Galliformes (Sub-orders: Mesites, Turnices, Galli, Opisthocomi), Gruiformes, Charadriiformes (Sub-orders: Limicolae, Lari, Pterocles, Columbae), Cuculiformes (Sub-orders: Cuculi, Psittaci), Coraciiformes (Sub-orders: Coraciae, Striges, Caprimulgi, Cypseli, Colii, Trogones, Pici), Passeriformes (Sub-orders: Passeres Anisomyodae, Passeres Diacromyodae).

Class 4. Mammalia (q.v.).

Sub-class 1. Monotremata (q.v.) (Prototheria).

No Orders.

Sub-class 2. Marsupialia (q.v.) (Metatheria).

One Order: Marsupialia.

Sub-orders: Polyprotodonta, Paucituberculata, Diprotodonta.

Sub-class 3. Placentalia (Monodelphia, q.v. or Eutheria).

Orders: Insectivora, Chiroptera, Dermoptera, Edentata (Sub-orders: Xenarthra, Pholidota, Tubulidentata), Rodentia (Sub-orders: Duplicidentata, Simplicidentata), Tillodontia, Carnivora (Sub-orders: Fissipedia, Pinnipedia, Creodonta), Cetacea (Sub-orders: Archaeoceti, Odontoceti, Mystacoceti), Sirenia, Ungulata (Sub-orders: Proboscidea, Hyracoidea, Barypoda, Toxodontia, Amblypoda, Litopterna, Ancylopoda, Condylarthra, Perissodactyla, Artiodactyla), Primates (Sub-orders: Prosimiae, Anthropoidea).

Phylum 14. Mesozoa (q.v.).

Class 1. Rhombozoa.

No Orders.

Class 2. Orthonectida.

No Orders.

Phylum 15. Polyzoa (q.v.).

Class 1. Entoprocta.

No Orders.

Class 2. Ectoprocta.

Orders: Gymnolaemata (Sub-orders: Tripostomata, Cryptostomata, Cyclostomata, Ctenostomata, Cheilostomata), Phylactolaemata.

Phylum 16. Acanthocephala (q.v.).

Class. Acanthocephala.

No Orders.

Phylum 17. Podaxonia.

Class 1. Sipunculoidea (q.v.).

No Orders.

Class 2. Priapuloidea (q.v.).

No Orders.

Class 3. Phoronidea (q.v.).

No Orders.

Class 4. Pterobranchia (q.v.).

No Orders.

Class 5. Brachiopoda (q.v.).

Sub-class 1. Ecardines (Inarticulata).

Orders: Atremata, Neotremata.

Sub-class 2. Testicardines (Articulata).

Orders: Protremata, Telotremata.

Phylum 18. Gastrotricha (q.v.).

Class. Gastrotricha.

Sub-orders: Ichthydina, Cepodina. (Possibly Kinorhyncha (q.v.) with only Echinoderes is to be placed here).

General Tendencies since Darwin

Darwin may be said to have founded the science of bionomics, and at the same time to have given new stimulus and new direction to morphography, physiology, and plasmology, by uniting them as contributories to one common biological doctrine—the doctrine of organic evolution—itself but a part of the wider doctrine of universal evolution based on the laws of physics and chemistry. The immediate result was, as pointed out above, a reconstruction of the classification of animals upon a genealogical basis, and an investigation of the individual development of animals in relation to the steps of their gradual building up by cell-division, with a view to obtaining evidence of their genetic relationships. On the other hand, the studies which occupied Darwin himself so largely subsequently to the publication of the Origin of Species, viz. the explanation of animal (and vegetable) mechanism, colouring, habits, &c., as advantageous to the species or to its ancestors, are only gradually being carried further. The most important work in this direction has been done by Fritz Müller (Für Darwin), by Herman Müller (Fertilization of Plants by Insects), by August Weismann (memoirs translated by Meldola) by Edward B. Poulton (see his addresses and memoirs published in the Transactions of the Entomological Society and elsewhere), and by Abbot Thayer (Concealing Coloration in the Animal Kingdom, Macmillan & Co. 1910). In the branch of bionomics, however, concerned with the laws of variation and heredity (thremmatology), there has been considerable progress. In the first place, the continued study of human population has thrown additional light on some of the questions involved, whilst the progress of microscopical research has given us a clear foundation as to the structural facts connected with the origin of the egg-cell and sperm-cell and the process of fertilization.

Great attention has been given lately to the important experiments upon the results of hybridizing certain cultivated varieties of plants which were published so long ago as 1865, by the Abbé Mendel, but failed to attract notice until thirty-five years later, sixteen years after his death (see Mendelism). Mendelism. Mendel’s object was to gain further knowledge as to result of mixing by cross-fertilization or interbreeding two strains exhibiting diverse characters or structural features. The whole question as to the mixture of characters in offspring thus produced was—and remains—very imperfectly observed. Mendel’s observations constitute an ingenious attempt to throw light on the matter, and in the opinion of some biologists have led to the discovery of an important principle. Mendel made his chief experiments with cultivated varieties of the self-fertilizing edible pea. He selected a variety with some one marked structural feature and crossed it with another variety in which that feature was absent. Instances of his selected varieties are the tall variety which he hybridized with a dwarf variety, a yellow-seeded variety which he hybridized with a green-seeded variety, and again a smooth-seeded variety which he hybridized with a wrinkle-seeded variety. In each set of experiments he concentrated his attention on the one character selected for observation. Having obtained a first hybrid generation, he allowed the hybrids to self-fertilize, and recorded the result in a large number of instances (a thousand or more) as to the number of individuals in the first, second, third and fourth generations in which the character selected for experiment made its appearance. In the first hybrid generation formed by the union of the reproductive germs of the positive variety (that possessing the structural character selected for observation) with those of the negative variety, it is not surprising that all or nearly all the individuals were found to exhibit, as a result of the mixture, the positive character. In subsequent generations produced by self-fertilization of the hybrids it was found that the positive character was not present in all the individuals, but that a result was obtained showing that in the formation of the reproductive cells (ova and sperms) of the hybrid, half were endowed with the positive character and half with the negative. Consequently the result of the haphazard pairing of a large number of these two groups of reproductive cells was to yield, according to the regular law of chance combination, the proportion 1PP, 2PN, 1NN, where P stands for the positive character and N for its absence or negative character—the positive character being accordingly present in three-fourths of the offspring and absent from one fourth. The fact that in the formation of the reproductive cells of the hybrid generation the material which carries the positive quality is not subdivided so as to give a half-quantity to each reproductive cell, but on the contrary is apparently distributed as an undivided whole to half only of the reproductive cells and not at all to the remainder, is the important inference from Mendel’s experiments. Whether this inference is applicable to other classes of cases than those studied by Mendel and his followers is a question which is still under investigation. The failure of the material carrying a positive character to divide so as to distribute itself among all the reproductive cells of a hybrid individual, and the limitation of its distribution to half only of those cells, must prevent the “swamping” of a newly appearing character in the course of the inter-breeding of those individuals possessed of the character with those which do not possess it. The tendency of the proportions in the offspring of 1PP, 2PN, 1NN is to give in a series of generations a regular reversion from the hybrid form PN to the two pure races, viz. the race with the positive character simply and the race with the total absence of it. It has been maintained that this tendency to a severance of the hybrid stock into its components must favour the persistence of a new character of large volume suddenly appearing in a stock, and the observations of Mendel have been held to favour in this way the views of those who hold that the variations upon which natural selection has acted in the production of new species are not small variations but large and “discontinuous.” It does not, however, appear that “large” variations would thus be favoured any more than small ones, nor that the eliminating action of natural selection upon an unfavourable variation could be checked.

A good deal of confusion has arisen in the discussions of this latter topic, owing to defective nomenclature. By some writers the word “mutation” is applied only to large and suddenly appearing variations which are found to be capable of hereditary transmission, whilst the term “fluctuation” is applied to small variations whether capable of transmission or not. By others the word “fluctuation” is apparently applied only to those small “acquired” variations due to the direct action of changes in food, moisture and other features of the environment. It is no discovery that this latter kind of variation is not hereditable, and it is not the fact that the small variations, to which Darwin attached great but not exclusive importance as the material upon which natural selection operates, are of this latter kind. The most instructive classification of the “variations” exhibited by fully formed organisms consists in the separation in the first place of those which arise from antecedent congenital, innate, constitutional or germinal variations from those which arise merely from the operation of variation of the environment or the food-supply upon normally constituted individuals. The former are “innate” variations, the latter are “superimposed” variations (so-called “acquired variations”). Both innate and superimposed variations are capable of division into those which are more and those which are less obvious to the human eye. Scarcely perceptible variations of the innate class are regularly and invariably present in every new generation of every species of living thing. Their greatness or smallness so far as human perception goes is not of much significance; their real importance in regard to the origin of new species depends on whether they are of value to the organism and therefore capable of selection in the struggle for existence. An absolutely imperceptible physiological difference arising as a variation may be of selective value, and it may carry with it correlated variations which appeal to the human eye but are of no selective value themselves. The present writer has, for many years, urged the importance of this consideration.

The views of de Vries and others as to the importance of “saltatory variation,” the soundness of which was still by no means generally accepted in 1910, may be gathered from the articles Mendelism and Variation. A due appreciation of the far-reaching results of “correlated variation” must, it appears, give a new and distinct explanation to the phenomena which are referred to as “large mutations,” “discontinuous variation” and “saltatory evolution." Whatever value is to be attached to Mendel’s observation of the breaking up of self-fertilized hybrids of cultivated varieties into the two original parent forms according to the formula “1PP, 2PN, 1NN,” it cannot be considered as more than a contribution to the extensive investigation of heredity which still remains to be carried out. The analysis of the specific variations of organic form so as to determine what is really the nature and limitation of a single “character” or “individual variation,” and whether two such true and strictly defined single variations of a single structural unit can actually “blend” when one is transmitted by the male parent and the other by the female parent, are matters which have yet to be determined. We do not yet know whether such absolute blending is possible or not, or whether all apparent blending is only a more or less minutely subdivided “mosaic” of non-combinable characters of the parents, in fact whether the combinations due to heredity in reproduction are ever analogous to chemical compounds or are always comparable to particulate mixtures. The attempt to connect Mendel’s observation with the structure of the sperm-cells and egg-cells of plants and animals has already been made. The suggestion is obvious that the halving of the number of nuclear threads in the reproductive cells as compared with the number of those present in the ordinary cells of the tissues—a phenomenon which has now been demonstrated as universal—may be directly connected with the facts of segregation of hybrid characters observed by Mendel. The suggestion requires further experimental testing, for which the case of the parthenogenetic production of a portion of the offspring, in such insects as the bee, offers a valuable opportunity for research.

Another important development of Darwin’s conclusions deserves special notice here, as it is the most distinct advance in the department of bionomics since Darwin’s own writings, and at the same time Variation.touches questions of fundamental interest. The matter strictly relates to the consideration of the “causes of variation,” and is as follows. The fact of variation is a familiar one. No two animals, even of the same brood, are alike: whilst exhibiting a close similarity to their parents, they yet present differences, sometimes very marked differences, from their parents and from one another. Lamarck had put forward the hypothesis that structural alterations acquired by (that is to say, superimposed upon) a parent in the course of its life are transmitted to the offspring, and that, as these structural alterations are acquired by an animal or plant in consequence of the direct action of the environment, the offspring inheriting them would as a consequence not unfrequently start with a greater fitness for those conditions than its parents started with. In its turn, being operated upon by the conditions of life, it would acquire a greater development of the same modification, which it would in turn transmit to its offspring. In the course of several generations, Lamarck argued, a structural alteration amounting to such difference as we call “specific” might be thus acquired. The familiar illustration of Lamarck’s hypothesis is that of the giraffe, whose long neck might, he suggested, have been acquired by the efforts of a primitively short -necked race of herbivores who stretched their necks to reach the foliage of trees in a land where grass was deficient, the effort producing a distinct elongation in the neck of each generation, which was then transmitted to the next. This process is known as “direct adaptation”; and there is no doubt that such structural adaptations are acquired by an animal in the course of its life, though such changes are strictly limited in degree and rare rather than frequent and obvious.

Whether such acquired characters can be transmitted to the next generation is a separate question. It was not proved by Lamarck that they can be, and, indeed, never has been proved by actual observation. Nevertheless it has been assumed, and also indirectly argued, that such acquired characters must be transmitted. Darwin’s great merit was that he excluded from his theory of development any necessary assumption of the transmission of acquired characters. He pointed to the admitted fact of congenital variation, and he showed that congenital variations are arbitrary and, so to speak, non-significant. Causes of congenital variation.Their causes are extremely difficult to trace in detail, but it appears that they are largely due to a “shaking up” of the living matter which constitutes the fertilized germ or embryo-cell, by the process of mixture in it of the substance of two cells—the germ-cell and the sperm-cell—derived from two different individuals. Other mechanical disturbances may assist in this production of congenital variation. Whatever its causes, Darwin showed that it is all-important. In some cases a pair of animals produce ten million offspring, and in such a number a large range of congenital variation is possible. Since on the average only two of the young survive in the struggle for existence to take the place of their two parents, there is a selection out of the ten million young, none of which are exactly alike, and the selection is determined hi nature by the survival of the congenital variety which is fittest to the conditions 'of life. Hence there is no necessity for an assumption of the perpetuation of direct adaptations. The selection of the fortuitously (fortuitously,Transmis-
sion of acquired and inherited characters. that is to say, so far as the conditions of survival are concerned) produced varieties is sufficient, since it is ascertained that they will tend to transmit those characters with which they themselves were born, although it is not ascertained that they could transmit characters acquired on the way through life. A simple illustration of the difference is this: a man born with four fingers only on his right hand is ascertained to be likely to transmit this peculiarity to some at least of his offspring; on the other hand, there is not the slightest ground for supposing that a man who has had one finger chopped off, or has even lost his arm at any period of his life, will produce offspring who are defective in the slightest degree in regard to fingers, hand or arm. Darwin himself, influenced by the consideration of certain classes of facts which seem to favour the Lamarckian hypothesis, was of the opinion that acquired characters are in some cases transmitted. It should be observed, however, that Darwin did not attribute an essential part to this Lamarckian hypothesis of the transmission of acquired characters, but expressly assigned to it an entirely subordinate importance.

The new attitude which has been taken since Darwin’s writings on this question is to ask for evidence of the asserted transmission of acquired characters. It is held^[10] that the Darwinian doctrine of selection of fortuitous congenital variations is sufficient to account for all cases, that the Lamarckian hypothesis of transmission of acquired characters is not supported by experimental evidence, and that the latter should therefore be dismissed. Weismann has also ingeniously argued from the structure of the egg-cell and sperm-cell, and from the way in which, and the period at which, they are derived in the course of the growth of the embryo from the egg—from the fertilized egg-cell—that it is impossible (it would be better to say highly improbable) that an alteration in parental structure could produce any exactly representative change in the substance of the germ or sperm-cells.

The one fact which the Lamarckians can produce in their favour is the account of experiments by Brown-Séquard, in which he produced epilepsy in guinea-pigs by section of the large nerves or spinal cord, and in the course of which he was led to believe that in a few rare instances the artificially produced epilepsy and mutilation of the nerves was transmitted. This instance does not stand the test of criticism. The record of Brown-Séquard’s original experiment is not satisfactory, and the subsequent attempts to obtain similar results have not been attended with success. On the other hand, the vast number of experiments in the cropping of the tails and ears of domestic animals, as well as of similar operations on man, are attended with negative results. No case of the transmission of the results of an injury can be produced. Stories of tailless kittens, puppies and calves, born from parents one of whom had been thus injured, are abundant, but they have hitherto entirely failed to stand before examination.

Whilst simple evidence of the fact of the transmission of an acquired character is wanting, the a priori arguments in its favour break down one after another when discussed. The very cases which are advanced as only to be explained on the Lamarckian assumption are found on examination and experiment to be better explained, or only to be explained, by the Darwinian principle. Thus the occurrence of blind animals in caves and in the deep sea was a fact which Darwin himself regarded as best explained by the atrophy of the organ of vision in successive generations through the absence of light and consequent disuse, and the transmission (as Lamarck would have supposed) of a more and more weakened and structurally impaired eye to the offspring in successive generations, until the eye finally disappeared. But this instance is really fully explained (as the present writer has shown) by the theory of natural selection acting on congenital fortuitous variations. It is definitely ascertained that many animals are thus born with distorted or defective eyes whose parents have not had their eyes submitted to any peculiar conditions. Supposing a number of some species of arthropod or fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would in the course of time escape, until only a pure race of eyeless or blind animals would be left in the cavern or deep sea.

It is a remarkable fact that it was overlooked alike by the supporters and opponents of Lamarck’s views until pointed out by the present writer (Nature, 1894, p. 127), that the two statements called by Lamarck his first and second laws are contradictory one of the other. Lamarck’s first law asserts that a past history of indefinite duration is powerless to create Educability. a bias by which the present can be controlled. He declares that in spite of long-established conditions and correspondingly evoked characters new conditions will cause new responsive characters. Yet in the second law he asserts that these new characters will resist the action of yet newer conditions or a reversion to the old conditions and be maintained by heredity. If the earlier characters were not maintained by heredity why should the later be? If a character of much longer standing (certain properties of height, length, breadth, colour, &c.) had not become fixed and congenital after many thousands of successive generations of individuals had developed it in response to environment, but gave place to a new character when new moulding conditions operated on an individual (Lamarck’s first law), why should we suppose that the new character is likely to become fixed and transmitted by mere heredity after a much shorter time of existence in response to environmental stimulus? Why should we assume that it will be able to escape the moulding by environment (once its evoking cause is removed) to which, according to Lamarck’s first law, all parts of organisms are subject? Clearly Lamarck gives us no reason for any such assumption, and his followers or latter-day adherents have not attempted to do so. His enunciation of his theory is itself destructive of that theory. Though an acquired or “superimposed” character is not transmitted to offspring as the consequence of the action of the external agencies which determine the “acquirement,” yet the tendency to react to such agencies possessed by the parent is transmitted and may be increased and largely developed by survival, if the character developed by the reaction is valuable. This newly discovered inheritance of “variation in the tendency to react” has a wide application and has led the present writer to coin the word “educability.” It has application to all kinds of organs and qualities, but is of especial significance in regard to the development of the brain and the mental qualities of animals and of man (see the jubilee volume of the Soc. de Biologie, 1899, and Nature, 1900, p. 624).

It has been argued that the elaborate structural adaptations of the nervous system which are the corporeal correlatives of complicated instincts must have been slowly built up by the transmission to offspring of acquiredTheory
of trans-
mission of instincts. experience, that is to say, of acquired brain structure. At first sight it appears difficult to understand how the complicated series of actions which are definitely exhibited as so-called “instincts” by a variety of animals can have been due to the selection of congenital variations, or can be otherwise explained than by the transmission of habits acquired by the parent as the result of experience, and continuously elaborated and added to in successive generations. It is, however, to be noted, in the first place, that the imitation of the parent by the young possibly accounts for some part of these complicated actions, and, secondly, that there are cases in which curiously elaborate actions are performed by animals as a characteristic of the species, and as subserving the general advantage of the race or species, which, nevertheless, can not be explained as resulting from the transmission of acquired experience, and must be supposed to be due to the natural selection of a fortuitously developed habit which, like fortuitous colour or form variation, happens to prove beneficial. Such cases are the habits of “shamming dead” and the combined posturing and colour peculiarities of certain caterpillars (Lepidopterous larvae) which cause them to resemble dead twigs or similar surrounding objects. The advantage to the animal of this imitation of surrounding objects is that it escapes the pursuit of (say) a bird which would, were it not deceived by the resemblance, attack and eat the caterpillar. Now it is clear that preceding generations of caterpillars cannot have acquired this habit of posturing by experience. Either the caterpillar postures and escapes, or it does not posture and is eaten; it is not half eaten and allowed to profit by experience. We seem to be justified in assuming that there are many movements of stretching and posturing possible to caterpillars, and that some caterpillars had a congenital fortuitous tendency to one position, some to another, and, finally that among all the variety of habitual movements thus exhibited one has been selected and perpetuated because it coincided with the necessary conditions of safety, since it happened to give the caterpillar an increased resemblance to a twig.

The view that instinct is the hereditarily fixed result of habit derived from experience long dominated all inquiry into the subject, but we may now expect to see a renewed and careful study of animal instincts carried out with the view of testing the applicability to each instance of the pure Darwinian theory without the aid of Lamarckism.

Nothing can be further from the truth than the once favourite theory that instincts are the survivals of lapsed reasoning processes. Instincts, or the inherited structural mechanisms of the nervous centres, are in antagonism to the results of the reasoning process, which are not capable of hereditary transmission. Every higher vertebrate animal possesses the power of forming for itself a series of cerebral mechanisms or reasoned conclusions based on its individual experience, in proportion as it has a large cerebrum and has got rid of or has acquired the power of controlling its inherited instincts. Man, The Record of the Past. compared with other animals, has the fewest inherited mental mechanisms or instincts and at the same time the largest cerebrum in proportion to the size of his body. He builds up, from birth onwards, his own mental mechanisms, and forms more of them, that is to say, is more “educable,” and takes longer in doing so, that is to say, in growing up and maturing his experience, than any other animal. The later stages of evolution leading from his ape-like ancestors to man have consisted definitely in the acquirement of a larger and therefore more educable brain by man and in the consequent education of that brain. A new and most important feature in organic development makes its appearance when we set out the facts of man’s evolutional history. It amounts to a new and unprecedented factor in organic development, external to the organism and yet produced by the activity of the organism upon which it permanently reacts. This factor is the Record of the Past, which grows and develops by laws other than those affecting the perishable bodies of successive generations of mankind, and exerts an incomparable influence upon the educable brain, so that man, by the interaction of the Record and his educability, is removed to a large extent from the status of the organic world and placed in a new and unique position, subject to new laws and new methods of development unlike those by which the rest of the living world is governed. That which we term the Record of the Past comprises the “taboos,” the customs, the traditions, the beliefs, the knowledge which are handed on by one generation to another independently of organic propagation. By it a new heredity, free from the limitations of protoplasmic continuity, is established. Its first beginnings are seen in the imitative tendencies of animals by which the young of one generation acquire some of the habits of their parents, and by which gregarious and social animals acquire a community of procedure ensuring the advantage of the group. “Taboo,” the systematic imposition by the community of restrictions upon the conduct of the individual, is one of its earliest manifestations in primitive man and can be observed even in animal communities. But with the development of the power of inter-communication by the use of language, the Record rapidly acquired an increased development, which was enormously extended by the continuous growth in mankind of the faculty of memory. To the mere tradition preserved by memory and handed on by speech was then added the written record and its later multiplication by the mechanical arts of printing, by which it acquired permanence and universal distribution. The result is the creation of an almost inconceivably vast body of traditional custom, law and knowledge into which every human being is born, less in the more isolated and barbarous communities, but large everywhere. Education is not in its essential nature a training administered to the young by an older generation, but is the natural and unaided assimilation of the Record of the Past by the automatically educable brain—an assimilation which is always in all races very large but becomes far larger in civilized communities. It is among them so important whilst the Record in all its details is so far beyond the receptive capacity of the brain, that selection and guidance are employed by the elders in order to enable the younger generation to benefit to the utmost by the absorption (so to speak) in the limited span of a lifetime of the most valuable influences to be acquired from this prodigious envelope of Recorded Experience. The imperishable Record invests the human race like a protective atmosphere, a new and yet a natural dispensation, giving to man, as compared with his animal ancestry, a new heaven and a new earth!

A result of the very greatest importance arising from the application of the generalizations of Darwinism to human development and to the actual phase of existing human population is that education has no direct effect upon the mental or physical features of the race or stock: it can only affect those of the individual. Educability, defects or excellences, or peculiarities of mind or body, can be handed on from parent to offspring by protoplasmic continuity in reproduction. But the results of education cannot be so handed on. The educated man who has acquired new experiences, new knowledge, can place these on the great Record for the benefit of future generations of men, but he cannot bodily transmit his acquirements to his offspring. Were acquired (superimposed) characters really transmissible by breeding, then every child born would inherit, more or less completely, the knowledge acquired by both its parents. But we know this is not the case: the child has to begin with a clean slate and learn for itself. Aptitudes and want of aptitude, which are innate and constitutional, are transmitted to offspring, but not the results of experience, education and training. Blemishes in the stock, defects of mind or body, though they may be to some extent corrected in the individual by training, cannot be got rid of from the stock by any such process. A defective stock, if allowed to breed, will perpetuate its defects, in spite of the concealment of those defects in an individual by training or other treatment. Equally it must be concluded that the weakness and degradation produced by semi-starvation and insanitary conditions of life are only an effect on the individual and cannot affect the stock. The stock may be destroyed, killed out by adverse conditions, but its quality is not directly affected, and if removed to more favourable conditions it will show no hereditary results of the previous adversity; indeed it will probably have been strengthened in some ways by the destruction in severe conditions of its weaker members and the survival of the stronger individuals. Such considerations have the very greatest importance for the guidance of the action of civilized man in seeking the health and happiness of the community. But it must not be forgotten that the problems presented by human communities are extremely complex, and that the absence of any selection of healthy or desirable stock in the breeding of human communities leads to undesirable consequences. The most thrifty and capable sections of the people at the present day are not (it has been shown) in overcrowded areas, producing offspring at such a rate as to contribute to the increase of the population. That increase, it has been shown, is due to the early marriage and excessive reproduction of the reckless and hopeless, the poorest, least capable, least desirable members of the community. The questions raised by these considerations have attracted much public attention under the newly invented name of “eugenics,” but they are of an exceedingly difficult and delicate nature.  (E. R. L.) 

1. The medieval attitude towards both plants and animals had no relation to real knowledge, but was part of a peculiar and in itself highly interesting mysticism. A fantastic and elaborate doctrine of symbolism existed which comprised all nature; witchcraft, alchemy and medicine were its practical expressions. Animals as well as plants were regarded as “simples” and used in medicine, and a knowledge of them was valued from this point of view.
2. The Academia Secretorum Naturae was founded at Naples in 1560, but was suppressed by the ecclesiastical authorities.
3. A very subtle and important qualification of this generalization has to be recognized (and was recognized by Darwin) in the fact that owing to the interdependence of the parts of the bodies of living things and their profound chemical interactions and peculiar structural balance (what is called organic polarity) the variation of one single part (a spot of colour, a tooth, a claw, a leaflet) may, and demonstrably does in many cases entail variation of other parts—what are called correlated variations. Hence many structures which are obvious to the eye, and serves as distinguishing marks of separate species, are really not themselves of value or use, but are the necessary concomitants of less obvious and even altogether obscure qualities, which are the real characters upon which selection is acting. Such “correlated variations” may attain to great size and complexity without being of use. But eventually they may in turn become, in changed conditions, of selective value. Thus in many cases the difficulty of supposing that selection has acted on minute and imperceptible initial variations, so small as to have no selective value, may be got lid of. A useless “correlated variation” may have attained great volume and quality before it is (as it were) seized upon and perfected by natural selection. All organisms are essentially and necessarily, built up by such correlated variations.
4. If we remember that by “blood” Aristotle understood “red blood,” and that he did not know of the existence of colourless blood, his primary division is not a bad one. One can imagine the interest and astonishment with which the great Greek would have been filled had some unduly precocious disciple shown to him the red-blood-system of the marine terrestrial Annelids; the red blood of Planorbis, of Apus cancriformis, and of the Mediterranean razor shell, Solen legumen.
5. The anatomical error in reference to the auricles of Reptiles and Batrachians on the part of Linnaeus is extremely interesting, since it shows to what an extent the most patent facts may escape the observation of even the greatest observers, and what an amount of repeated dissection and unprejudiced attention has been necessary before the structure of the commonest animals has become known.
6. Sir Edwin Ray Lankester (b. 1847) was the eldest son of Edwin Lankester (1814–1874), a physician and naturalist (F.R.S. 1845), who became well known as a scientific writer and lecturer, editor of the Quarterly Journal of Microscopical Science from 1853 to 1871, and from 1862, in succession to Thomas Wakley, coroner for Central Middlesex. Educated at St Pad’s and both at Downing College, Cambridge, and Christ Church, Oxford, E. Ray Lankester obtained the Radcliffe Travelling Fellowship at Oxford in 1870, and became a fellow and lecturer at Exeter College in 1872. From 1874 to 1890 he was professor of zoology and comparative anatomy at University College, London; and from 1891 to 1898 Linacre professor of comparative anatomy at Oxford. From 1898 to 1907 he was director of the Natural History Department of the British Museum. He was made K.C.B. in 1907. [Ed. E. B.].
7. Ursprung der Wirbelthiere (Leipzig, 1875); and Lankester, Degeneration (London, 1880).
8. It is to be noted that the terms used for designating categories in the classification are not always identical in this summary and separate articles, as authors differ as to the use of these.
9. Craniata may be usefully divided into 3 grades: (a) Branchiata Hetcrodactyla, which includes Pisces except Cyclostomes. (b) Branchiata Pentadactyla, which includes Batrachia. (c) Lipobranchia Pentadactyla, which includes Reptiles, Birds and Mammals.
10. Weismann, Vererbung, &c. (1886).