Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity.

I shall have to return to this subject in the chapter on classification, but I may add that as, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and, as all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms.

Although few of the most ancient species have left modified descendants, yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders and classes, as at the present day.
ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.
Natural selection acts exclusively by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions to which each creature is exposed at all periods of life.
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.