XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 891 



veloped types — exactly such a change as we may expect to 

 find if the evokition theory be the true one. Many other 

 illustrations of a similar change could be given, but the 

 animal groups in which they occur being less familiar, the 

 details Avould be less interesting, and perhaps hardly intel- 

 ligible. There is, however, one very remarkable proof of 

 development that must be briefly noticed — that aflbrded by 

 the steady increase in the size of the brain. This may be 

 best stated in the words of Professor Marsh : — 



" The real progress of mammalian life in America, from 

 the beginning of the Tertiary to the present, is well illus- 

 trated by the brain-gi'owth, in which we have the key to 

 many other changes. The earliest known Tertiary mammals 

 all had very small brains, and in some forms this organ was 

 proiDortionally less than in certain reptiles. There was a 

 gradual increase in the size of the brain during this period, 

 and it is interesting to find that this growth was mainly 

 confined to the cerebral hemispheres, or higher portion of the 

 brain. In most gi'oups of mammals the brain has gradually 

 become more convoluted, and thus increased in quality as 

 well as quantity. In some also the cerebellum and olfactory 

 lobes, the lower parts of the brain, have even diminished in 

 size. In the long struggle for existence during Tertiary time 

 the big brains won, then as now ; and the increasing power 

 thus gained rendered useless many structures inherited from 

 primitive ancestors, but no longer adapted to new conditions." 



This remarkable proof of development in the organ of 

 the mental faculties, forms a fitting climax to the evidence 

 already adduced of the progressive evolution of the general 

 structure of the body, as illustrated by the bony skeleton. 

 We now pass on to another class of facts equally suggestive 

 of evolution. 



The Local Relations of Fossil and Living Animals. 



If all existing animals have been produced from ancestral 

 forms — mostly extinct — under the law of variation and natural 

 selection, we may expect to find in most cases a close rela- 

 tion between the living forms of each country and those which 

 inhabited it in the immediately preceding epoch. But if 

 species have originated in some quite difterent way, either by 



