ORIGIN OF MAMMALIA 57 
beginnings, approximate more and more towards a central type 
which is most nearly represented among known fossils by the 
earliest Creodonta (Oxyclenide) of the dawn of the Tertiary. 
The Insectivora and Rodentia are also groups of mammals which 
in most respects have not departed very far from this primitive 
type. Its general characters are, (1) Small size; (2) Small brain 
of low organization; (3) Forty-four teeth of simple construction, 
with sharp cusps, the molars, premolars, canines and incisors of 
different form, the molars having the “tritubercular’’ pattern ; 
(4) Limbs and neck flexible and of moderate length, tail very 
long and powerful, probably prehensile; (5) Feet with five digits 
on each foot, claws on the toes, the thumb more or less opposable. 
These characters appear to indicate an arboreal mode of life 
rather than any other, and we may suppose that during the Age 
of Reptiles the ancestors of the mammals were tree-living ani- 
mals, feeding chiefly upon insects. They were insignificant in 
size and unimportant in numbers, quite overshadowed by the 
great and numerous reptilian fauna which flourished during that 
long era. They possessed, however, the two most important ele- 
ments of final success in the evolutionary struggle; a brain 
which, though inferior to that of their descendants, was superior 
to the brain of all other contemporary vertebrates, and a con- 
struction of the joints of limbs and feet more mechanically per- 
fect than in any other animals. By the further improvement and 
elaboration of these factors of success, they were enabled to dis- 
place all their rivals, and become dominant upon land and to 
some extent upon the sea. Their invasion of the aérial province, 
already occupied by the highly developed and specialized birds, 
has been less successful, but of the once dominant reptile fauna 
of the land, almost nothing remains. The triumphant mammals 
have branched and re-branched, diverged into countless special- 
izations in adaptation to peculiar modes of life, some of which 
have survived, while others have become extinct, but always the 
prime factors of success in the long run have been those which 
gave them their original advantage over their reptile competi- 
tors. Finally the truth that the supremacy in intelligence is 
first in importance, is best illustrated by the present dominance 
of man over the whole terrestrial world. 
