Charles Darwin gave new stimulus and new direction to morphology and physiology, by uniting them as part of a common biological theory: the theory of organic evolution but a part of the wider doctrine of universal evolution based on the laws of physics and chemistry. The immediate result was, a reconstruction of the classification of animals upon a genealogical basis, and an investigation of the individual development of animals, and early attempts to determine their genetic relationships.
Warning: this is largely based on the 1911 Encyclopedia Brittannica, and contains several errors
Early 20th century work
The studies which occupied Darwin himself subsequent to the publication of the Origin of Species, that is the explanations of animal and plant mechanisms, coloring, habits, which confer advantages to the individuals within a species, were only gradually being carried further in the early 20th century. Much important work in this direction was 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 field of what would become known as genetics, the laws of variation and heredity (originally known as thremmatology), there was considerable progress during this period. The progress of microscopy during this era began to give a clearer understanding of the structural facts connected with the origin of the egg-cell and sperm-cell and the process of fertilization.
Mendel and zoology
Gregor Mendel's experiments hybridizing certain cultivated varieties of plants were published in 1865, but failed to attract notice until thirty-five years later in the early 20th century, sixteen years after his death (see Mendelism). Mendel's object was to gain a better understanding of the principles of heredity.
Mendel made his chief experiments with cultivated varieties of the self-fertilizing edible pea. He selected a variety with 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 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 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 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 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 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 Hugo 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 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 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, as a 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 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 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, has 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; 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.
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 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, 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 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 Darwin's writings on this question is to ask for evidence of the asserted transmission of acquired characters. It is held 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. August 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 bisection 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 tailess 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 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 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, etc.) 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 acquired 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 (Lepidoptera 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.
Record of the past
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 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 acquisition 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 evolutionary 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.