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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 prim- 

 itive 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 generation, 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 greater importance. 

 For such missing links, we must look to the caves and later 

 Tertiary of Africa, which I regard as now the most promising 

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

 Tropics, can promise no such inducements to ambitious ex- 

 plorers. 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 discov- 

 eries 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 Selection, 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 important contest with the 

 elements, and all surrounding nature. By changes in the envi- 

 ronment, migrations are enforced, slowly in some cases, rapidly 

 in others, and with change of locality must come adaptation to 

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

 mammals illustrates this principle at every stage, and no other 

 explanation meets the facts. 



The real progress of mammalian life in America, from the 

 beginning of the Tertiary to the present, is well illustrated by 

 the Brain-growth, in which we have the key to many other 

 changes. /The earliest known Tertiary mammals all had very 



