Зображення сторінки


separate organism as the expression of powers and tendencies not to be accounted for by “Natural Selection” alone, or even by that together with merely the direct influence of surrounding conditions.

The difficulties which appear to oppose themselves to the reception of "Natural Selection “the survival of the fittest," as the one explanation of the origin of species, have no doubt been already considered by Mr. Darwin. Nevertheless, it may be worth while to enumerate them, and to state the considerations which appear to give them weight; and there is no doubt but that a naturalist so candid and careful as the author of the theory in question, will feel obliged, rather than the reverse, by the suggestion of all the difficulties which can be brought against it.

What is to be brought forward may be summed up as follows:

That “Natural Selection” is incompetent to account for the incipient stages of useful structures.

That it does not harmonize with the co-existence of closely similar structures of diverse origin.

That there are grounds for thinking that specific differences may be developed suddenly instead of gradually.

That the opinion that species have definite though very different limits to their variability is still tenable.

That certain fossil transitional forms are absent, which might have been expected to be present.

That some facts of geographical distribution intensify other difficulties.

That the objection drawn from the physiological diference between “species” and “races” still exists unrufated,

That there are many remarkable phenomena in organic forms upon which “Natural Selection” throws no light whatever, but the explanations of which, if they could be attained, might throw light upon specific origination.

Besides these objections to the sufficiency of “Natural Selection,” others may be brought against the hypothesis of “Pangenesis," which, professing as it does to explain great difficulties, seems to do so by presenting others not less great-in fact almost to be the explanation of obscurum per obscurius.





Mr. Darwin supposes that Natural Selection acts by slight variations.

These must be useful at once.- Difficulties as to the giraffe ; as to mimicry; as to the heads of flat-fishes ; as to the origin and constancy of the vertebrate limbs; as to whalebone ; as to the young kangaroo ; as to sea-urchins ; as to certain processes of metamorphosis; as to the mammary gland; as to certain ape characters; as to the rattlesnake and cobra ; as to the process of formation of the eye and ear; as to the fully developed condition of the eye and ear; as to the voice ; as to shell-fish ; as to orchids ; as to ants. — The necessity for the simultaneous modification of many individuals. -Summary and conclusion.

“ NATURAL Selection," simply and by itself, is potent to explain the maintenance or the further extension and development of favourable variations, which are at once sufficiently considerable to be useful from the first to the individual possessing them. But Natural Selection utterly fails to account for the conservation and development of the minute and rudimentary beginnings, the slight and insignificant commencements of structures, however useful those structures may afterwards become.

Now, it is distinctly enunciated by Mr. Darwin, that the spontaneous variations upon which his theory depends are individually slight, minute, and insensible.

1 “Animals and Plants under Domestication,” vol. ii.

He says,

[ocr errors]


[ocr errors]

Slight individual differences, however, suffice for the work, and are probably the sole differences which are effective in the production of new species.” And again, after mentioning the frequent sudden appearances of domestic varieties, he speaks of “the false belief as to the similarity of natural species in this respect.” 1 In his work on the Origin of Species,” he also observes, “Natural Selection acts only by the preservation and accumulation of small inherited modifications." 2 And “Natural Selection, if it be a true principle, will banish the belief . . . of any great and sudden modification in their structure.”3 Finally, he adds, “If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." 4

Now the conservation of minute variations in many instances is, of course, plain and intelligible enough; such, e.g., as those which tend to promote the destructive faculties of beasts of prey on the one hand, or to facilitate the flight or concealment of the animals pursued on the other; provided always that these minute beginnings are of such a kind as really to have a certain efficiency, however small, in favour of the conservation of the individual possessing them; and also provided that no unfavourable peculiarity in any other direction accompanies and neutralizes, in the struggle for life, the minute favourable variation.

[ocr errors]


“Animals and Plants under Domestication,” vol. ii. p. 414.
2 “Origin of Species,” 5th edit., 1869, p. 110.
3 Ibid. p. 111.

4 Ibid. p. 227. Even in his recently published work, Mr. Darwin observes, "Slight fluctuating differences in the individual suffice for the work of natural selection.” See “Descent of Man,” vol. ii. p. 387.

But some of the cases which have been brought forward, and which have met with very general acceptance, seem less satisfactory when carefully analysed than they at first appear to be. Amongst these we may mention “the neck of the giraffe.”

At first sight it would seem as though a better example in support of “Natural Selection” could hardly have been chosen. Let the fact of the occurrence of occasional, severe droughts in the country which that animal has inhabited be granted. In that case, when the ground vegetation has been consumed, and the trees alone remain, it is plain that at such times only those individuals (of what we assume to be the nascent giraffe species) which were able to reach high up would be preserved, and would become the parents of the following generation, some individuals of which would, of course, inherit that highreaching power which alone preserved their parents. Only the high-reaching issue of these high-reaching individuals would again, cæteris paribus, be preserved at the next drought, and would again transmit to their offspring their still loftier stature; and so on, from period to period, through aons of time, all the individuals tending to revert to the ancient shorter type of body, being ruthlessly destroyed at the occurrence of each drought.

(1.) But against this it inay be said, in the first place, that the argument proves too much ; for, on this supposition, many species must have tended to undergo a similar modification, and we ought to have at least several forms, similar to the giraffe, developed from different Ungulata:

i The order Ungulata contains the hoofed beasts; that is, all oxen, deer, antelopes, sheep, goats, camels, hogs, the hippopotamus, the different kinds of rhinoceros, the tapirs, horses, asses, zebras, quaggas, &c.

« НазадПродовжити »