hypothesis that the retrogression of the eyes of cavern-dwelling animals is at all due to increased survival caused by diversion of nutriment to more useful organs, is untenable. Moreover, the case of the eye is quite an exceptional one; it is an extreme example. Much less can we suppose that the retrogression of other organs is due to economy of nutrition. For instance, as regards the toes of horses; the modern horse has only one toe on each limb, his ancestors had five. While the transition from five toes to one was occurring, thousands of generations lived and died, and tens of thousands of years elapsed. If economy of nutrition, in this case, led to the retrogression of the four toes which are now absent, by influencing the survival rate, what was the amount of nutriment saved for the other organs by those individuals that varied favourably? Such an amount only as could have been supplied by an extra mouthful—I had almost said an extra blade—of grass per diem. It is therefore doubly plain that this theory will not bear examination, and that Mr. Spencer is right in ridiculing it.
As regards the theory that the retrogression of useless organs is due to the effect that injury to them exercises on the chances of survival, here again the eye is an extreme example. No other organ in the body can be thought of as delicate and important which can be as easily injured, and therefore the retrogression of no other organ can be as readily affected by this means.
To take the example given by Mr. Spencer: "Suppose that in this new habitat the kangaroo had no enemies; and suppose that, consequently, quickness of hearing not being called for, large ears gave no greater advantage than small ones, would an individual with smaller ears than usual survive and propagate better than other individuals, in consequence of his ears being less exposed to injury? Assuredly not."
