The objections which have been urged against the theory of natural selection fall into two classes.
I. Those which strike at its root, which either deny that there is any natural selection, or declare that it is not capable of producing a new species.
II. Those which are directed against the all-sufficiency of natural selection to account for organic evolution.
Those of the first class need not detain us long, although among those who formulate them are to be found some eminent men of science.
Delage alleges that selection is powerless to form species, its function is, according to him, limited to the suppression of variations radically bad, and to the maintaining of a species in its normal character. It is thus an inimical factor in evolution, a retarder rather than an accelerator of species-change. It merely acts by preserving the type at the expense of the variants, and so acts as a brake on evolution.
Korschinsky, while possibly not denying that selection occurs in nature, declares that its influence on evolution is nil, or, if it has any influence, that it is a hindering one.
Eimer similarly denies any capacity on the part of natural selection to create species.
Pfeffer urges a very different objection. He says that if such a force as natural selection existed it would transform species much more rapidly than it does!
Now, in order that the above objections can carry any weight, one of two sets of conditions must be fulfilled.
Either all organisms must be perfectly adapted to their environment, and this environment must never change, or there must be inherent in each species a kind of growth-force which impels the species to develop in certain fixed directions. In either of these circumstances natural selection will be an inhibitory force, for if the normal organism is perfectly adapted to its environment, all variations from the type must be unfavourable, and natural selection will weed out the individuals that display them. No careful student of nature can maintain, either that all animals are perfectly adapted to their environment, or that this never changes. Hence those who deny that natural selection is a factor in the making of species, assume the second set of conditions, that species develop in certain fixed directions, being impelled either by internal or external forces. How far these ideas are founded on fact we shall endeavour to determine when speaking of variation. It must suffice at present to say that even if any of these views of orthogenesis be established, natural selection will have, so to speak, a casting vote, it will decide which series of species developing along preordained lines shall survive and which shall not survive.