Origin of Species Variorum

modification of some kind. The existence of such places will often depend on physical changes, which are generally very slow, and on the immigration of better adapted forms having been checked. ↑ But the action of natural selection will probably still oftener depend on some few of the inhabitants becoming slowly modified; the mutual relations of many of the other inhabitants being thus disturbed. Nothing can be effected, unless favourable variations occur, and variation itself is apparently always a .. slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so. On the other hand, I do believe that natural selection always acts very slowly, generally at only long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. I further believe, that this slow, intermittent action of natural selection accords perfectly .. with what geology tells us of the rate and manner at which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection.

Extinction caused by Natural Selection.

This subject will be more fully discussed in our chapter on Geology; but it must be here alluded to from being intimately connected with natural selection.