The reader will best understand what is meant, if he will take the trouble to refer to the diagram in the fourth chapter.

We will suppose the letters A to L to represent allied genera existing during the Silurian epoch, and descended from some still earlier form.

In three of these genera (A, F, and I) a species has transmitted modified descendants to the present day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal line.

Now, all these modified descendants from a single species are related in blood or descent in the same degree.

They may metaphorically be called cousins to the same millionth degree, yet they differ widely and in different degrees from each other.