Every species, so long as the conditions under which it lives remain the same, becomes more and more perfectly fitted to those conditions in every generation.

But now suppose those conditions change, then the sort of individual that used to succeed may now fail to succeed and a sort of individual that could not get on at all under the old conditions may now find its opportunity. These species will change, therefore, generation by generation; the old sort of individual that used to prosper and dominate will fail and die out and the new sort of individual will become the rule, — until the general character of the species chan