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. [Illustration: 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.