Let us take a group of birds, descended from some ancient form and modified through natural selection for different habits.

Then, from the many slight successive variations having supervened in the several species at a not early age, and having been inherited at a corresponding age, the young will have been but little modified, and they will still resemble each other much more closely than do the adults, just as we have seen with the breeds of the pigeon.

We may extend this view to widely distinct structures and to whole classes.