| If both have become fitted for slightly different habits of life or conditions, they might live together; though, in the case of animals which freely cross and wander much about, varieties seem to be almost always confined to distinct localities.
But if we put on one side polymorphic species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size, albinism, &c., the more permanent varieties are generally found, as far as I can judge, inhabiting distinct stations, high land or low land, dry or moist districts, or distinct regions.
Bronn also insists that distinct species never differ from each other only in single characters, but in many parts; and he asks, how it comes that natural selection should invariably have affected simultaneously many parts of the organisation?
But there is not the least necessity for believing that all the parts have been simultaneously modified; they may have been gained one after the other, and from being transmitted together, they appear to us as if simultaneously formed.
Correlation, however, will account for various parts changing, when any one part changes.
We have evidence of this in our domestic races, which though they may differ greatly in some one selected character, always differ to a certain extent in other characters.
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