Изображения страниц
PDF
EPUB

a

pendently at a conclusion practically identical with Darwin's. On the other hand, Darwin is entitled to all the credit which should belong to an investigator who tried and tested his hypothesis in every possible way, and who shrank from submitting it to the public until it had been fortified with an overwhelming array of supporting facts, and had been well exposed to its author's own unsparing criticism.

The conception of natural selection in its bearing on evolution, save for Wallace's independent discovery, is Darwin's own. Such intimations of the principle as had occurred to other thinkers, as to Mr. Patrick Matthew, were never followed up and consequently came to nothing; in Darwin's hands the mere suggestion furnished the key to the whole problem. It must not be forgotten that Darwin had convinced himself of the fact of evolution some years before he was put on the track of natural selection by his study of the famous treatise of Malthus, and Mr. Francis Darwin has expressed the opinion that ' with his knowledge of the interdependence of organisms and the tyranny of conditions, his experience would have crystallised out into “ a theory by which to work” even without the aid of Malthus.'

However this may be, it is interesting to inquire how far his confidence in the fact of evolution was a plant of spontaneous growth. He himself attributed the rise of doubts in his mind regarding the permanence of species to his observations during the voyage of H.M.S. Beagle of certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent.' There is no evidence to shew that he had made himself acquainted with the views of previous advocates of evolution ; but it is difficult to believe that he had never heard of the transformist speculations of his grandfather, Erasmus Darwin. Still we have his own testimony that at any rate when starting on the voyage in the Beagle in 1831 he believed in the permanence of specific forms; and on the other hand, it is an indisputable fact that in the July of 1837 he opened his first note-book on the transmutation of species. It has been justly pointed out both by Huxley and by Mr. Francis Darwin that Lyell's Principles of Geology, the first volume of which was put into Darwin's hands by Henslow in 1831, by its teaching of geological uniformitarianism ‘leads inevitably to evolution.'

When in due course Darwin took occasion to read Lamarck, he was not favourably impressed with that author's general line of argument. 'Heaven forfend me,' he wrote to Hooker in 1844, * from Lamarck nonsense of a

tendency to progression," "adaptations from the slow willing of animals," &c. ! But the conclusions I am led to are not widely different from his; though the means of change are wholly so.' Notwithstanding his generally poor

' opinion of Lamarck's speculations, he retained to the last a belief in the French naturalist's tenet of the transmissibility of acquired characters.

But, as we have already said, the great distinctive feature of Darwin's contribution to evolutionary theory is his application thereto of the principle of selection, and it is by the wealth of argument and illustration which he brought to bear on the problems involved in the operation of this principle that he makes his strongest appeal to the scientific thought of to-day. Various attempts have been made, all of them, as we think, unsuccessful, to dispense with natural selection as a dominant factor in the evolutionary process; but we venture to assert that it is becoming more and more clearly recognized that the only alternative to natural selection as an explanation of adaptation to conditions is a return to the old hypothesis of ‘ special creation'

a in a form that is neither warranted by science nor welcomed by theology.

Much attention of late years has centred round the question of the nature and causes of variation. Darwin, as is well known, took the fact of variation as a starting point, without discussing at any length the problem of its origin. Besides the ordinary fluctuations about a mean, the existence of which kind of variation is a matter of common observation, he recognized the occasional occurrence of 'sports' or variations of a more pronounced kind, such in fact as may fairly take rank as abnormalities. Variations

[ocr errors]

of this latter description might, he thought, serve for the foundation of domestic races; as an instance of such a result he gives the case of the Ancon or 'otter-sheep,' a breed which, originating from a definite sport, was preserved for some generations by means of artificial selection. But he was not inclined to allow any importance to the occurrence of sports in regard to species-formation under natural con. ditions. Without rigid selection, such as can be applied by man, the extreme variations would, he considered, speedily be obliterated by crossing. This opinion of Darwin's has now been vigorously contested, and (as we shall see) with some measure of success. It was shewn many years ago by Gregor Mendel, Abbot of Brünn, that divergent characters are not necessarily blended or swamped when the bearers of them are crossed with one another. It is true that a unitcharacter of one parent may so predominate over the corresponding character in the other parent that the first generation of the offspring may shew the former only, to the apparent exclusion of the latter. But when the members of this first generation are allowed to breed together, a certain proportion of the resulting offspring is found to present the character which was apparently missing in their parents, though present in one grandparent. This establishes the fact that certain characters may remain latent in one generation and reappear in the next. There has, of course, long been a general consensus of opinion to this effect; but before Mendel the phenomenon had never been put on a scientific footing. The proportion in which the latent or 'recessive' character reappears was found by Mendel to be constant, and on the facts observed by him as a result of his experiments he formulated his now famous law of the segregation of unit-characters in the germ-cells or 'gametes. Into this part of the subject we need not enter further than to remark that Mendel's discovery renders necessary a revision of Darwin's opinion as to the swamping effects of intercrossing, inasmuch as it has now been clearly shewn that in many cases at all events two divergent characters do not in reality counteract or blend with one another when crossed, but may each of them reappear in all VOL. LXVIII.---NO. CXXXVI.

Е Е

their original purity in successive generations. Hence the question of the significance of 'sports,' or as they are now often called, 'mutations,' demands a more extended treatment than that accorded to it by Darwin. It is manifest that if a 'mutation and its parent form stand to one another in the Mendelian' relation, there will be no swamping, and the mutation will at least have its chance. But it must still run the gauntlet of selection, either natural or artificial ; and whether Darwin was right or wrong in minimizing the importance of mutations in species-formation, there is absolutely no warrant for limiting the sway of his distinctive principle over every sort and kind of variation.

We may conjecture with some degree of safety that if Darwin had been acquainted with Mendel's work, and had traced out Mendel's results to their legitimate consequences, he would have been led to admit, at least as a theoretical possibility, that sports might take part in the formation of new species. But it is difficult, or even impossible, to believe that he would be found on the side of de Vries and his followers at the present day, in discarding the fluctuating variations altogether as having any species-forming significance, and in attributing to mutations, and mutations alone, the power of becoming the ancestors of a new organic type. The successive accumulation of small differences is an essential part of the evolutionary process as conceived by both Darwin and Wallace, and the statement for which Professor Hubrecht has lately made himself responsible, that 'de Vries is a much more staunch Darwinian than Wallace himself,' appears to us to be almost grotesque in its absurdity.

A point which is of the first importance in the study of organized nature is the well-nigh universal occurrence of adaptation. Every naturalist worthy of the name has from the earliest times been impressed with the fact that the objects of his interest are not a mere array of isolated and unrelated phenomena, but are rather constituents in a great unified complex, no one part of which could be interfered with without disturbing the balance of the whole. It is not easy to find any feature, whether in animal or plant,

which has not some assignable relation to the needs of its possessor in relation to its surroundings either inorganic or organic. The number of exceptions to this rule, already comparatively insignificant, is diminishing almost daily. There is no need to insist upon a fact which has been universally recognized by observers from the time of Aristotle and earlier. No one in modern times has dealt more sympathetically with this great outstanding feature of organized nature than has Paley in his Evidences; and it is interesting to note that Darwin (whose rooms, by the way, at Christ's College, Cambridge, were once, according to tradition, occupied by Paley himself) attributed to the study of the Evidences the only good which he believed he derived from the official studies of his University. Whatever else the Evidences may have done or failed to do for him, they helped to direct his mind into a channel which led to the most fruitful consequences; and we have no hesitation in asserting that in proportion as anyone occupies himself, like Darwin, in enlarging his acquaintance with the contrivances,' as Paley called them, of Nature, to that extent will he find himself constrained to accept the Darwinian view of their development, and to recognize the inadequacy of the 'mutationist hypothesis to account for their existence.

According to the school of de Vries, the mutation, when it appears, is already a new species ; nor can new species arise in any other way. When confronted with the problem of adaptation, and asked how it is that new species, which arrive, according to the hypothesis, per saltum, should nevertheless be so perfectly adapted to their surroundings, the mutationists can only answer that selection has here been at work, and that those mutations alone survive which can stand the test of competition. But to ask us to believe that the nice adjustments which we see everywhere around us represent merely the survivors of mutations which appeared suddenly as such from dissimilar progenitors, is to put too great a strain on our credulity. It will be generally felt that if the doctrine of special creation ’is to be revived, it should not be in this shape. No such difficulty attends the Darwin-Wallace conception of the accumulation of minute

[ocr errors]
« ПредыдущаяПродолжить »