Passing from these difficulties, the other great leading facts in palaeontology agree admirably with the theory of descent with modification through variation and natural selection.

We can thus understand how it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent.

The extinction of old forms is the almost inevitable consequence of the production of new forms.

We can understand why, when a species has once disappeared, it never reappears.