On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, "Natura non facit saltum.

" This canon, if we look to the present inhabitants alone of the world, is not strictly correct; but if we include all those of past times, whether known or unknown, it must on this theory be strictly true.

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.