September 9, 1898.] 



SCIENCE. 



321 



affairs until the very highest Cretaceous. 

 At the close of the Mesozoic the area of 

 the peneplain was uplifted and there came 

 into it the new life. Not onlj' the changed 

 geographic conditions, but the better-fitted 

 mammalia, also were probably factors in 

 terminating the life of the peneplains."* 



After the placental mammals once be- 

 came established, as the result of favorable 

 geographical conditions, of migrations, iso- 

 lation, and secondarily of competition, the 

 evolution as well as the elimination of 

 forms, as is well known, went on most 

 rapidly. Remains of over two thousand 

 species of extinct mammals during Tertiary 

 times which existed in America north of 

 Mexico have been already described, where 

 at present there are scarcely more than 

 three hundred. This process of specializa- 

 tion involved not only the lengthening of 

 the legs, the change from plantigrade to 

 digitigrade, and to limbs adapted for seiz- 

 ing and handling their prey or food, or for 

 swimming and climbing ; the reduction of 

 digits ; the evolution of armatures, pro- 

 tective scales, etc.; but, above all, an in- 

 crease in the mental capacity of the later 

 forms, not only of mammals, but of birds, 

 as shown by the progressive increase in size 

 of their brains ; those of certain existing 

 mammals being eight times as large, in 

 proportion to the bulk of the body, as 

 those of their early Tertiary ancestors. 

 This, of course, means that aaimal shrewd- 

 ness, cunning and other intellectual quali- 

 ties, the result of semi social attrition and 

 competition, had begun to displace the 

 partly physical factors, and in the primates 

 these may have in the beginning led to the- 

 appearance of man, a social animal, with 

 the power of speech, and all the intelligent 

 moral and spiritual qualities, which per- 

 haps primarily owe their genesis to increas- 

 ed brain-power. 



* American Geologist, Vol. XIV., Oct., 1894, pp. 

 208-235. 



The three most specialized types of mam- 

 mals below men are the horse, the bats and 

 the whales. In the case of the bats, which 

 appear in the Eocene, Nature's experiment 

 with these mammalian aeronauts succeeded 

 to the extent that they still exist in small 

 numbers. Late in the Cretaceous, or very 

 early in the Eocene, competition apparently 

 forced some unknown carnivorous type to 

 take up an aquatic life, and the great suc- 

 cess of the incoming cetacean type, result- 

 ing in the Eocene zeuglodonts and Miocene 

 Squalodon, may have had an influence on 

 the final extinction of the colossal marine 

 reptiles. 



6. THE QUATERNARY PERIOD. 



Coming now to the glacial epoch of the 

 Quaternary period, we plainly see that, 

 under the extreme conditions to which as 

 never before, life in the northern hemisphere 

 was exposed how intimate are the relations 

 of geology and biology. 



The rise of land at the beginning of the 

 Quaternary, which carried the land and 

 the life on it up into a cooler zone, with 

 a mean temperature so low that the 

 snows remained from century to century 

 unmelted, forming continental glaciers, 

 excited an immediate influence on the life. 

 There were very soon developed a circum- 

 polar flora and fauna, originating from the 

 few Pliocene forms which became adapted 

 to climatic conditions more extreme than 

 ever before known in the world's history. 

 While a few forms thus survived, some 

 must have perished, though the bulk of them 

 migrated southward. 



The story told by the Port Kennedy hole, 

 in Pennsylvania, just south of the limits of 

 the ice sheet, is a most striking one. In 

 that assemblage where are intermingled the 

 bones of mammals of the Appalachian sub- 

 province, with certain extinct forms, and 

 those of the tapir and peccary and colossal 

 sloths,adapted to the warmth of the Pliocene 



