Let us see how far these several facts and inferences accord with the theory of descent with modification.

As the subject is somewhat complex, I must request the reader to turn to the diagram in the fourth chapter.

We may suppose that the numbered letters in italics represent genera, and the dotted lines diverging from them the species in each genus.

The diagram is much too simple, too few genera and too few species being given, but this is unimportant for us.

The horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct.