This article considers the following areas:
It must recognize the following five branches of zoological study:
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 nameswhich arc 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, etc.; Zoo-dynamics under medicine, surgery, physiology, anatomy, embryology, and allied articles; Plasmology under cytology, protoplasm, etc.; and Philosophical Zoology under numerous headings, evolution, bioloigy, etc.
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 iu the study of zoology since Darwin.
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 today 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 Edward Wotton, born at Oxford in 1492, who practised Wotton. as a physician in London and died in 1555. He published a treatise De differentiis animalium at Paris in 1552. In many respects Wotton 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 Wottons 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 Aristotle's classificatory conceptions of successive naturalists classifi- with those which are to be found in the works of cAldon.
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 morphegraphy, physiology, and plasmology, by uniting them as contrbutories to one common biological doctrinethe doctrine of organic evolutionitself 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 ef 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, etc., 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 Muller (Für Darwin), by Herman Muller (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 I 865, by the Abbé Mendel, but failed to attract notice until thirty-five years later, sixteen years after his death (see Mendelism[?]). Mendel- Mendels object was to gain further knowledge as to Ism, the 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 wasand remainsvery imperfectly observed. Mendels 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 1 PP, 2 PN, 1 NN, 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 Mendels 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 1 PP, 2 PN, 1 NN 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 01 natural selection upon a.n 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 he 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 arc 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 a~s 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 appeals, 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 Mendels observation of the breaking up of self-fertilized hybrids of cultivated varieties into the two original parent forms according to the formula 1 PP, 2 PN, 1 NN, 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 Mendels observation with the structure of the spermcells 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 tissuesa phenomenon which has now been demonstrated as universal may he directly connected with the facts of segregation of hybrid characters observed by Mendel. The suggestion r&luires 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 Darwins conclusions deserves special notice here, as it is the most distinct advance Var! a- in the department of bionomics since Darwins own tion, writings, and at the sa1ne time 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 Lamarcks 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 Larnarck 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. Darwins 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 Their causes are extremely difficult to trace in detail, congeni- but it appears that they are largely due to a shaking talvar!a- 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 cellsthe germ. cell and the sperm-cellderived 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 in 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, Transthat is to say, so far as the conditions of survival are mission concerned) produced varieties is sufficient, since it is ascertained that they will tend to transmit those characters with which they themselves were born, herited although it is not ascertained that they could transmit charcharacters 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 Darwins writings on this question is to ask for evidence of the asserted transmission of acquired characters. It is held f that the Darwinian doctrine of selection of fortuitous congenital variations is sufficient to account for all cases, that the Lamarckian hypothesis of transmission cf 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 eggfrom the fertilized egg-cellthat 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-Séquards 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 he 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 Lamarcks 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. Lamarcks first Jaw asserts that a past history of indefinite duration is powerless to create Ethica- a bias by which the present can be controlled. He bility. declares that in spite of long-established conditions and correspondingly evoked characters new conditions will cause new responsive characters. Yet in the second Ltw 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 (Lamarcks 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 Lamarcks 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 oltrans- up by the transmission to offspring of acquired cxmission perience, 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 Inovements 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, compared with other animals, has the fewest inherited Record mental mechanisms or instincts and at the same time the the largest cerebrum in proportion to the size of his Past. 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 leadifig 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 mans 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,
Much of the contents of this article are taken from the 1911 Encyclopaedia Britannica.