The difficulties which appear to oppose themselves to the reception of "Natural Selection" or "the survival of the fittest," as the one explanation of the origin of species, have no doubt been already considered by Mr. Darwin. Nevertheless, it may be worth while to enumerate them, and to state the considerations which appear to give them weight; and there is no doubt but that a naturalist so candid and careful as the author of the theory in question, will feel obliged, rather than the reverse, by the suggestion of all the doubts and difficulties which can be brought against it.
What is to be brought forward may be summed up as follows:—
That "Natural Selection" is incompetent to account for the incipient stages of useful structures.
That it does not harmonize with the co-existence of closely similar structures of diverse origin.
That there are grounds for thinking that specific differences may be developed suddenly instead of gradually.
That the opinion that species have definite though very different limits to their variability is still tenable.
That certain fossil transitional forms are absent, which might have been expected to be present.
That some facts of geographical distribution supplement other difficulties.
That the objection drawn from the physiological difference between "species" and "races" still exists unrefuted.
That there are many remarkable phenomena in organic forms upon which "Natural Selection" throws no light whatever, but the explanations of which, if they could be attained, might throw light upon specific origination.