Page:Darwinism by Alfred Wallace 1889.djvu/399



theory of evolution in the organic world necessarily implies that the forms of animals and plants have, broadly speaking, progressed from a more generalised to a more specialised structure, and from simpler to more complex forms. We know, however, that this progression has been by no means regular, but has been accompanied by repeated degradation and degeneration; while extinction on an enormous scale has again and again stopped all progress in certain directions, and has often compelled a fresh start in development from some comparatively low and imperfect type.

The enormous extension of geological research in recent times has made us acquainted with a vast number of extinct organisms, so vast that in some important groups—such as the mollusca—the fossil are more numerous than the living species; while in the mammalia they are not much less numerous, the preponderance of living species being chiefly in the smaller and in the arboreal forms which have not been so well preserved as the members of the larger groups. With such a wealth of material to illustrate the successive stages