This is important, because it affords ample time for great changes of conditions since the structures in question originated; and without a knowledge of these changes we can never safely assert that any detail of structure could not have been useful.

In the second place, all three are cases of aborted or rudimentary organs; and these are admitted to be explained by non-use, leading to diminution of size, a further reduction being brought about by the action of the principle of economy of growth.

But, when so reduced, the rudiment might be inconvenient or even hurtful, and then natural selection would aid in its complete abortion; in other words, the abortion of the part would be _useful_, and would therefore be subject to the law of survival of the fittest.
The genera Ateles and Colobus are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless.

The case of the Potto is more difficult, both because it is, presumably, a more ancient type, and its actual life-history and habits are completely unknown.

These cases are, therefore, not at all to the point as proving that positive specific characters--not mere rudiments characterising whole genera--are in any case useless.
Mr.Mivart further objects to the alleged rigidity of the action of natural selection, because wounded or malformed animals have been found which had evidently lived a considerable time in their imperfect condition.