‘The second case is that detailed by a no less unexceptionable authority than Réaumur, in his Art de faire éclore les Poulets. A Maltese couple named Kelleia, whose hands and feet were constructed upon the ordinary human model, had born to them a son, Gratio, who possessed six perfectly moveable fingers on each hand, and six toes, not quite so well formed, on each foot. No cause could be assigned for the appearance of this unusual variety of the human species. But however they may have arisen, what especially interests us is to remark that, once in existence, varieties obey the fundamental law of reproduction, that like tends to produce like, and their offspring exemplify it by tending to exhibit the same deviation from the parental stock as themselves. Indeed, there seems to be in many instances a prepotent influence about a newly arisen variety which gives it what we may call an unfair advantage over the normal descendants from the same stock. This is strikingly exemplified by the case of Gratio Kelleia, who married a woman with the ordinary pentadactyle extremities and had by her four children, Salvator, George, André, and Marie. Of these children Salvator, the eldest boy, had six fingers and six toes, like his father; the second and third, also boys, had five fingers and five toes, like their mother, though the hands and feet of George were slightly deformed; the last, a girl, had five fingers and five toes, but the thumbs were slightly deformed. The variety thus reproduced itself purely in the eldest, while the normal type reproduced itself purely in the third, and almost purely in the second and last; so that it would seem, at first, as if the normal type were more powerful than the variety. But all these children grew up and intermarried with normal wives and husbands, and then note what took place—Salvator had four children, three of whom exhibited the hexadactyle members of their grandfather and father, while the youngest had the pentadactyle limbs of the mother and grandmother; so that here, notwithstanding a double pentadactyle dilution of the blood the hexadactyle variety had the best of it. The same prepotency of the variety was still more markedly exemplified in the progeny of two of the other children, Marie and George. Marie (whose thumbs only were deformed) gave birth to a boy with six toes, and three other normally formed children; but George, who was not quite so pure a pentadactyle, begot, first, two girls, each of whom had six fingers and toes; then a girl with six fingers on each hand, and six toes on the right foot, but only five toes on the left; and lastly, a boy with only five fingers and toes. In these instances, therefore, the variety, as it were, leaped over one generation to reproduce itself in full force in the next. Finally, the purely pentadactyle André was the father of many children, not one of whom departed from the normal parental type.’
169. The instances now quoted illustrate two things. Both tell us how varieties arise, we may say spontaneously, or in other words we cannot tell how; and the former instance, that of the Ancon breed, shows us moreover that such varieties when they do occur may be rendered permanent by means of artificial selection. If the six-fingered descendants of Gratio Kelleia had been forced to intermarry amongst themselves it is highly probable that we should have had a permanent hexadactyle variety of the human race. It has likewise been shown by Charles Darwin that the pouter, the fan-tail, the carrier, and the tumbler are all varieties of the common rock-pigeon.
170. It thus appears that permanent varieties may be produced by artificial selection. Now Darwin and Wallace have brought before us the very great fact that similar changes can also be produced by natural selection.
To illustrate this, let us imagine a slight variety to arise spontaneously, we do not know how. Having arisen there is a ‘prepotent influence’ about it which enables it to secure a considerable proportion of offspring having its own characteristics. Now, suppose that the characteristics are such as to adapt the individuals possessing them more perfectly to the conditions of nature which surround them. When, by breeding amongst themselves, the new variety is rendered permanent, the members of this variety will, therefore, have an advantage over their elder brethren so far as certain conditions of nature are concerned, will in fact succeed better in the struggle for existence, and will ultimately displace the elder branches. Thus the struggle for existence bears to natural selection the same relation as man bears to artificial selection.
171. We now come to the real point of difficulty, or at least the unproved point, in the Darwinian hypothesis. We may cross one race with another, but we do not obtain, so far as we know, those phenomena of infertility which are exhibited when we cross distinct species with each other. The Ancon sheep were perfectly fertile when matched with their elder brethren, and the dray-horse and the Arab, or the pouter and the tumbler, breed together as easily as if they were of the same race. But if we cannot produce infertility, how can we apply the results of artificial selection to account for the origin of species?
This difficulty is met by Darwin and his followers in this way:—‘It is not as yet proved,’ says Professor Huxley, ‘that a race ever exhibits, when crossed with another race of the same species, those phenomena of hybridisation which are exhibited by many species when crossed with other species. On the other hand, not only is it not proved that all species give rise to hybrids infertile inter se, but there is much reason to believe that, in crossing, species exhibit every gradation from perfect sterility to perfect fertility.’ This appears to carry weight; the old theory went with a leap from perfect fertility to perfect sterility, and did not contemplate the possibility of a continuous gradation from the one extreme to the other; at least its argument was founded upon the neglect of such a gradation. But if there be a gradation of this kind, it follows that infertility will merely represent the results of crossing two species whose functional characteristics are very different from each other; and, on the other hand, the reason why artificially produced varieties are not infertile when crossed with one another may only be that the experiment has not been continued long enough.
Time, in fact, is the essential requisite in all such attempts to imitate nature.
172. In connection with this subject, Mr. Darwin has remarked that certain plants are more fertile with the pollen of another species than with their own; and Professor Huxley tells us that there are certain fuci whose male element will fertilise the ovule of a plant of distinct species, while the males of the latter species are ineffective with the females of the first. So obscure in some of its branches is the working of the reproductive system.
Again, the following remark by Mr. Darwin is very suggestive:—
‘First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication, by the selection of mere external differences, and not of differences in the reproductive system. In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels.’