CHAPTEE XIII 



THE GEOLOGICAL EVIDENCES OF EVOLUTION 



Wliat we may expect — The number of known species of extinct animals — 

 Causes of the imperfection of the geological record — Geological 

 evddences 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, progi^essed 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 



