In this case it would be quite impossible to give definitions by which the several members of the several groups could be distinguished from their more immediate parents and descendants.

Yet the arrangement in the diagram would still hold good and would be natural; for, on the principle of inheritance, all the forms descended, for instance from A, would have something in common.

In a tree we can distinguish this or that branch, though at the actual fork the two unite and blend together.

We could not, as I have said, define the several groups; but we could pick out types, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them.