It is generally acknowledged that all organic beings have been formed on two great laws--Unity of Type, and the Conditions of Existence.

By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life.

On my theory, unity of type is explained by unity of descent.

The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection.