Origin of Species Variorum

of climate itself, would .. seriously affect .. the others. If the country were open on its borders, new forms would certainly immigrate, and this also would often seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same places would have been seized .. by intruders. In such cases, .. slight modifications, which in ... any way favoured the individuals of any .. species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would .. have free scope for the work of improvement.

We have reason to believe, as stated in the first chapter, that .. changes in the conditions of life cause or excite a tendency to vary; and in the foregoing case the conditions are supposed to have changed, and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring; and unless such do occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere individual differences are always included. As man can certainly produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily, from having incomparably longer time for action. ↑ Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigra- tion,