Thus, earthworms are adapted to their mode of life better than they would be if more highly organised.

So, in the ocean, the minute foraminifera and infusoria, and the larger sponges and corals, occupy places which more highly developed creatures could not fill.

They form, as it were, the base of the great structure of animal life, on which the next higher forms rest; and though in the course of ages they may undergo some changes, and diversification of form and structure, in accordance with changed conditions, their essential nature has probably remained the same from the very dawn of life on the earth.

The low aquatic diatomaceae and confervae, together with the lowest fungi and lichens, occupy a similar position in the vegetable kingdom, filling places in nature which would be left vacant if only highly organised plants existed.

There is, therefore, no motive power to destroy or seriously to modify them; and they have thus probably persisted, under slightly varying forms, through all geological time.
_Extinction of Lower Types among the Higher Animals._ So soon; however, as we approach the higher and more fully developed groups, we see indications of the often repeated extinction of lower by higher forms.