I believe that the ARRANGEMENT of the groups within each class, in due subordination and relation to each other, must be strictly genealogical in order to be natural; but that the AMOUNT of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by the forms being ranked under different genera, families, sections or orders.

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.