Fig. 8.—Rudimentary or vestigial hind-limbs of Python, as exhibited in the skeleton and on the external surface of the animal. Drawn from nature, ¼ nat. size (Zoological Gardens).
Now, rudimentary organs of this kind are of such frequent occurrence, that almost every species presents one or more of them—usually, indeed, a considerable number. How, then, are they to be accounted for? Of course the theory of descent with adaptive modification has a simple answer to supply—namely, that when, from changed conditions of life, an organ which was previously useful becomes useless, it will be suffered to dwindle away in successive generations, under the influence of certain natural causes which we shall have to consider in future chapters. On the other hand, the theory of special creation can only maintain that these rudiments are formed for the sake of adhering to an ideal type. Now, here again the former theory appears to be triumphant over the latter; for, without waiting to dispute the wisdom of making dwarfed and useless structures merely for the whimsical motive assigned, surely if such a method were adopted in so many cases, we should expect that in consistency it would be adopted in all cases. This reasonable expectation, however, is far from being realized. We have already seen that in numberless cases, such as that of the fore-limbs of serpents, no vestige of a rudiment is present. But the vacillating policy in the matter of rudiments does not end here; for it is shown in a still more aggravated form where within the limits of the same natural group of organisms a rudiment is sometimes present and sometimes absent. For instance, although in nearly all the numerous species of snakes there are no vestiges of limbs, in the Python we find very tiny rudiments of the hind-limbs. (Fig. 8.) Now, is it a worthy conception of Deity that, while neglecting to maintain his unity of ideal in the case of nearly all the numerous species of snakes, he should have added a tiny rudiment in the case of the Python—and even in that case should have maintained his ideal very inefficiently, inasmuch as only two limbs, instead of four, are represented? How much more reasonable is the naturalistic interpretation; for here the very irregularity of their appearance in different species, which constitutes rudimentary structures one of the crowning difficulties to the theory of special design, furnishes the best possible evidence in favour of hereditary descent; seeing that this irregularity then becomes what may be termed the anticipated expression of progressive dwindling due to inutility. Thus, for example, to return to the case of wings, we have already seen that in an extinct genus of bird, Dinornis, these organs were reduced to such an extent as to leave it still doubtful whether so much as the tiny rudiment hypothetically supplied to Fig. 6 (p. [61]) was present in all the species. And here is another well-known case of another genus of still existing bird, which, as was the case with Dinornis, occurs only in New Zealand. (Fig. 9.) Upon this island there are no four-footed enemies—either existing or extinct—to escape from which the wings of birds would be of any service. Consequently we can understand why on this island we should meet with such a remarkable dwindling away of wings.
Fig. 9.—Apteryx Australis. Drawn from life in the Zoological Gardens, 1/8 nat. size. The external wing is drawn to a scale in the upper part of the cut. The surroundings are supplied from the most recent descriptions.
Similarly, the logger-headed duck of South America can only flap along the surface of the water, having its wings considerably reduced though less so than the Apteryx of New Zealand. But here the interesting fact is that the young birds are able to fly perfectly well. Now, in accordance with a general law to be considered in a future chapter, the life-history of an individual organism is a kind of condensed recapitulation of the life-history of its species. Consequently, we can understand why the little chickens of the logger-headed duck are able to fly like all other ducks, while their parents are only able to flap along the surface of the water.
Facts analogous to this reduction of wings in birds which have no further use for them, are to be met with also in insects under similar circumstances. Thus, there are on the island of Madeira somewhere between 500 and 600 species of beetles, which are in large part peculiar to that island, though related to other—and therefore presumably parent—species on the neighbouring continent. Now, no less than 200 species—or nearly half the whole number—are so far deficient in wings that they cannot fly. And, if we disregard the species which are not peculiar to the island—that is to say, all the species which likewise occur on the neighbouring continent, and therefore, as evolutionists conclude, have but recently migrated to the island,—we find this very remarkable proportion. There are altogether 29 peculiar genera, and out of these no less than 23 have all their species in this condition.
Similar facts have been recently observed by the Rev. A. E. Eaton with respect to insects inhabiting Kerguelen Island. All the species which he found on the island—viz. a moth, several flies, and numerous beetles—he found to be incapable of flight; and therefore, as Wallace observes, “as these insects could hardly have reached the islands in a wingless state, even if there were any other known land inhabited by them, which there is not, we must assume that, like the Madeiran insects, they were originally winged, and lost their power of flight because its possession was injurious to them"—Kerguelen Island being “one of the stormiest places on the globe,” and therefore a place where insects could rarely afford to fly without incurring the danger of being blown out to sea.
Here is another and perhaps an even more suggestive class of facts.
It is now many years ago since the editors of Silliman’s Journal requested the late Professor Agassiz to give them his opinion on the following question. In a certain dark subterranean cave, called the Mammoth cave, there are found some peculiar species of blind fishes. Now the editors of Silliman’s Journal wished to know whether Prof. Agassiz would hold that these fish had been specially created in these caves, and purposely devoided of eyes which could never be of any use to them; or whether he would allow that these fish had probably descended from other species, but, having got into the dark cave, gradually lost their eyes through disuse. Prof. Agassiz, who was a believer in special creation, allowed that this ought to constitute a crucial test as between the two theories of special design and hereditary descent. “If physical circumstances,” he said, “ever modified organized beings, it should be easily ascertained here.” And eventually he gave it as his opinion, that these fish “were created under the circumstances in which they now live, within the limits over which they now range, and with the structural peculiarities which now characterise them.”