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.

It might be thought that the amount of change which the various parts and organs pass through in their development from embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva.

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