Page - 95 - in The Origin of Species

NATURAL SELECTION 95 of the planets? Every one knows what is meant and is im- plied by such metaphorical expressions ; and they are almost necessary for brevity. So again it is difficult to avoid per- sonifying the word Nature; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten. We shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The propor- tional numbers of its inhabitants will almost immediately un- dergo a change, and some species will probably become ex- tinct. We may conclude, from what we have seen of the in- timate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the rela- tions of some of the former inhabitants. Let it be remem- bered 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 are'a been open to immigration, these same places would have been seized on 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 good reason to believe, as shown in the first chap- ter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the con- ditions have changed, and this would manifestly be favour- able to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, nat- ural selection can do nothing. Under the term of "vari-