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.

This is what we should be driven to, if we were ever to succeed in collecting all the forms in any one class which have lived throughout all time and space.

Assuredly we shall never succeed in making so perfect a collection: nevertheless, in certain classes, we are tending toward this end; and Milne Edwards has lately insisted, in an able paper, on the high importance of looking to types, whether or not we can separate and define the groups to which such types belong.