VERTEBRATE LIFE IN AMERICA. 695 



closely the corresponding parts of the highest Old World apes, than 

 do the latter our Tertiary Primates, or even the recent American 

 monkeys. Various living and fossil forms of Old World Primates fill 

 up essentially the latter gap. The lesser gap between the primitive 

 man of America and the Anthropoid apes is partially closed by still 

 lower forms of men, and doubtless also by higher apes, now extinct. 

 Analogy, and many facts as well, indicate that this gap was smaller 

 in the past. It certainly is becoming wider now with every genera- 

 tion, for the lowest races of men will soon become extinct, like the 

 Tasmanians, and the highest apes cannot long survive. Hence the 

 intermediate forms of the past, if any there were, become of still great- 

 er importance. For such missing links, we must look to the caves 

 and later Tertiary of Africa, which I regard as now the most promis- 

 ing field for exploration in the Old World. America, even in the 

 tropics, can promise no such inducements to ambitious explorers. We 

 have, however, an equally important field, if less attractive, in the 

 Cretaceous mammals, which must have left their remains somewhere 

 on this continent. In these two directions, as I believe, lie the most 

 important future discoveries in paleontology. 



As a cause for many changes of structure in mammals during the 

 Tertiary and Post-Tertiary, I regard as the most potent, natural selec- 

 tion, in the broad sense in which that term is now used by American 

 evolutionists. Under this head I include not merely a Malthusian 

 struggle for life among the animals themselves, but the equally im- 

 portant contest with the elements and all surrounding Nature. By 

 changes in the environment, migrations are enforced, slowly in some 

 cases, rapidly in others, and with change of locality must come adap- 

 tation to new conditions, or extinction. The life-history of Tertiary 

 mammals illustrates this principle at every stage, and no other expla- 

 nation meets the facts. 



The real progress of mammalian life in America, from the begin- 

 ning of the Tertiary to the present, is well illustrated by the brain- 

 growth, in which we have the key to many other changes. The ear- 

 liest known Tertiary mammals all had very small brains, and in some 

 forms this organ was proportionally 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 con- 

 fined to the cerebral hemispheres, or higher portion of the brain. In 

 most groups of mammals, the brain has gradually become more con- 

 voluted, 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 dur- 

 ing Tertiary time, the big brains won, then as now; and the increas- 

 ing power thus gained rendered useless many structures inherited 

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



