Origin of Species Variorum

the most favoured or improved varieties will have been enabled to spread: there will have been much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the reunited continent will again have been changed; and again there will have been a fair field for natural selection to improve still further the inhabitants, and thus to produce new species.

That natural selection .. acts with extreme slowness I fully admit. The result depends on there being places in the polity of nature, which can be better filled through some of the inhabitants of the country under-going modifications 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 being checked. But the effects of natural selection will probably still oftener depend on some few of the inhabitants becoming slowly modified; the mutual relations of the other inhabitants being thus disturbed. ↑ Although all the individuals of the same species differ more or less from each other, ... differences of the right nature, better adapted to the then existing conditions, may not soon occur. The results will often be greatly retarded by free inter-crossing. ↑ Many will exclaim that these several causes are amply sufficient .. to neutralise the power of natural selection. I do not believe so. But I do believe that natural selection generally acts very slowly in effecting changes, at long intervals of time, and only on .. a .. few of the inhabitants of the same region. .. .. .. .. I further believe that these slow, intermittent results of natural selection accord 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