We have as yet only spokesn incidentally of the disappearance of species and of groups of species.

On the theory of natural selection, the extinction of old forms and the production of new and improved forms are intimately connected together.

The old notion of all the inhabitants of the earth having been swept away by catastrophes at successive periods is very generally given up, even by those geologists, as Elie de Beaumont, Murchison, Barrande, etc.

, whose general views would naturally lead them to this conclusion.

On the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world.