ORIGIN OF MAMMALIA 39 



beginnings, approximate more and more towards a central type 

 which is most nearly represented among known fossils by the 

 earliest Creodonta (Oxyclsnidas) 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, (i) 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 aerial 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. Fmally 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. 



