The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most.

And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,--varieties of the same species, and species of the same genus or related genera,--which, from having nearly the same structure, constitution and habits, generally come into the severest competition with each other.

Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them.

We see the same process of extermination among our domesticated productions, through the selection of improved forms by man.

Many curious instances could be given showing how quickly new breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and inferior kinds.