Moreover, the ancient forms of life are often what is called "comprehensive types"-- that is to say, they possess characters in combination such as we nowadays only find separately developed in different, groups of animals.

Now, this permanent retention of embryonic characters and this "comprehensiveness" of structural type are signs of what a zoologist considers to be a comparatively low grade of organisation; and the prevalence of these features in the earlier forms of animals is a very striking phenomenon, though they are none the less perfectly organised so far as their own type is concerned.

As we pass upwards in the geological scale, we find that these features gradually disappear, higher and ever higher forms are introduced, and "specialisation" of type takes the place of the former comprehensiveness.

We shall have occasion to notice many of the facts on which these views are based at a later period, and in connection with actual examples.

In the meanwhile, it is sufficient to state, as a widely-accepted generalisation of palaeontology, that there has been in the past a general progression of organic types, and that the appearance of the lower forms of life has in the main preceded that of the higher forms in point of time..