670 THE AMERICAN NATURALIST. (VOL; XXXII. 
reptilia of the peneplain. Their development seems also to have been 
retarded.” Again he says: “ To sum up the faunal history of the Mesozoic 
alone, we have seen that pari passu with the creation of broad lowlands 
there was brought on to the stage a remarkable production of reptiles, a 
characteristic lowland life; and we note that the humble mammalia were 
excluded from the peneplain or held back in their development, so far as 
we know them by actual remains, during this condition of 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 only 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 became established, as 
the result of favorable geographical conditions of migrations, 
isolation, 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 spe- 
cialization involved not only the lengthening of the legs, the 
change from plantigrade to digitigrade, and to limbs adapted 
for seizing and handling their prey or food, or for swimming 
and climbing; the reduction of digits; the evolution of arma- 
tures, protective scales, etc.; but above all’ an increase 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 
animal shrewdness, cunning, and other intellectual qualities, 
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 intel- 
ligent, moral, and spiritual qualities, which, perhaps, primarily 
owe their genesis to increased brain power. 
The three most specialized types of mammals below men 
are the horse, the bats, and the whales. In the case of the 
1 American Geologist, vol. xiv (October, 1894), pp- 209-235- 
