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.