The larger and more dominant groups within each class thus tend to go on increasing in size, and they consequently supplant many smaller and feebler groups.

Thus, we can account for the fact that all organisms, recent and extinct, are included under a few great orders and under still fewer classes.

As showing how few the higher groups are in number, and how widely they are spread throughout the world, the fact is striking that the discovery of Australia has not added an insect belonging to a new class, and that in the vegetable kingdom, as I learn from Dr.Hooker, it has added only two or three families of small size.
In the chapter on geological succession I attempted to show, on the principle of each group having generally diverged much in character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some degree intermediate between existing groups.

As some few of the old and intermediate forms having transmitted to the present day descendants but little modified, these constitute our so-called osculant or aberrant groups.

The more aberrant any form is, the greater must be the number of connecting forms which have been exterminated and utterly lost.