phrase into the error of arguing in a circle; for, in the case of not a few creatures which have survived, it is difficult to give any good reason for their survival except upon the assumption of their fitness as proved by the very fact of their survival. Thus their fitness leads to their survival, and this survival leads to the conclusion that they must have been the fittest. Which is arguing in a circle. Still further, it is not difficult to suggest examples in which the expression, survival of the fittest, manifestly breaks down. Sir Isaac Newton was, as is well known, a very delicate child, difficult to rear. Suppose that Newton and a powerful navvy, or coal porter, or grenadier, had been compelled to rough it as children at Dotheboys Hall or some similar establishment, which would have survived? Not Newton; and yet it may be fairly argued that in many respects he would have been the fittest. Nor is this imaginary case an altogether unfair test of the propriety of the phrase; for it is impossible to give any true definition of fitness which shall exclude all moral and intellectual qualities, all qualities in fact which are of the highest value, and which shall simply include those elements of toughness and wiriness, and strength of sinew or stomach, which are chiefly calculated to prolong life in trying circumstances.
Putting out of consideration, however, the propriety of the language by which survival in the struggle for life, whether among vegetables or animals, is expressed, it is to be admitted that the principle indicated is a true one. That is to say, it may be regarded as admitted by all persons whose studies and natural powers render their opinion of any real value, that modification by natural selection is an element in that evolution of living forms of which the evidence appears to be irresistible. Natural selection is a vera causa; the question is, What is the extent of its action? how much can it do?
Darwin considered it necessary to supplement natural by that which he termed sexual selection; in doing which he was quite consistent, because he speaks (as we have already seen) of natural selection as "the most important, but not the exclusive, means of modification" of species. This supplemental hypothesis, however, does not commend itself to Mr. Wallace's judgment.
Again, after explaining his own views on the subject of ornamental appendages of birds and other animals, he writes: