CHAPTER XIII 
THE GEOLOGICAL EVIDENCES OF EVOLUTION 
What we may expect—The number of known species of extinct animals— 
Causes of the imperfection of the geological record—Geological 
evidences of evolution—Shells—Crocodiles—The rhinoceros tribe— 
The pedigree of the horse tribe—Development of deers’ horns—Brain 
development—Local relations of fossil and living animals—-Cause of 
extinction of large animals—Indications of general progress in plants 
and animals—The progressive development of plants—Possible cause 
of sudden late appearance of exogens—Geological distribution of 
insects—Geological succession of vertebrata—Concluding remarks. 
The theory of evolution in the organic world necessarily im¬ 
plies 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 
