Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/868

Rh 818 ZOOLOGY is the most distinct advance in the department of biono mics since Darwin s own writings, and at the same time touches questions of fundamental interest. The matter strictly relates to the consideration of the &quot; causes of varia tion,&quot; and is as follows. Varia- The fact of variation is a familiar one. No two animals, tion. even O f t k e 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 hypo thesis that structural alterations acquired by 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 &quot;specific &quot; 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 &quot; direct adaptation;&quot; and there is no doubt that such structural adaptations are acquired by an animal in the course of its life. 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 imist be transmitted. Darwin s great merit was that he excluded from his theory of develop ment any necessary assumption of the transmission of ac quired characters. He pointed to the admitted fact of congenital variation, and he showed that these variations to all intents and purposes have nothing to do with any characters acquired by the parents, but are arbitrary and, Causes of so to speak, non-significant. Their causes are extremely congen- difficult to trace in detail, but it appears that they are al vari &quot; largely due to a &quot; shaking up &quot; of the living matter which constitutes the fertilized germ or embryo-cell, by the pro cess 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 Trans- of direct adaptations. The selection of the fortuitously mission (fortuitously, that is to say, so far as the conditions of sur- Qufred Vivs ^ are concerned) produced varieties is sufficient, since it and in- is ascertained that they will tend to transmit those char- herited acters 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 differ ence is this : a man born with four fingers only on his right ation. char acters. 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, apparently influenced not merely by the consideration of certain classes of facts which seem to favour the Lamarckian hypothesis but also by a respect for the general prejudice in its favour and for Mr Herbert Spencer s authority, was of the opinion that acquired characters are in some cases transmitted. It should be observed, however, that Darwin did not attribute an es sential part to this Lamarckian hypothesis of the trans mission of acquired characters, but expressly assigned to it an entirely subordinate importance. The new attitude which has been taken since Darwin on this question is to ask for evidence of this 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 char acters 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. It does not seem improbable that the doctrine of organic evolution will thus become pure Darwinism and be entirely dissociated from the Lamarckian hereby. The one fact which the Lamarckians can produce in their favour is the account of experiments by Brown-Sequard, 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 was transmitted. This in stance does not stand the test of criticism. It is not clear whether the guinea-pigs operated upon had or had not already a constitutional tendency to epilepsy, and it is not clear in what proportion of cases the supposed transmission took place, and whether any other disease accompanied it. 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 re sults 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. Experimental researches on this question are most urgently needed, but they are not provided for either in the morphographical or physiological laboratories of our universities. 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 ex plained, 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 1 Weismann, Vererbuny, &c., 1886.