The ultimate result is that each creature tends to become more and more improved in relation to its conditions.

This improvement inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world.

But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation.

Among the vertebrata the degree of intellect and an approach in structure to man clearly come into play.