Class Mammalia 567 



Incisors were strongly developed and sharp-edged. There were no 

 teeth in the position of canines. The molars were large and surmounted 

 by numerous pointed protuberances or "cusps" — wherefore the group 

 is called Multituberculata. The group persisted through the Meso- 

 zoic and into the early part of the Cenozoic Age. 



Taken all together, the established facts indicate that, in Triassic 

 times and along a line of relatively small and somewhat primitive 

 reptiles, mammalian characteristics were gradually acquired. Toward 

 the latter part of this period and for reasons at present not definitely 

 known, the reptiles which were merely "mammal-like" in some par- 

 ticulars perished, while certain of their contemporaries which had 

 acquired more nearly the full complement of mammalian character- 

 istics survived. Most important for survival were the more efficient 

 locomotor mechanism, in all probability some degree of warm-blooded- 

 ness and control of body temperature associated with possession of 

 hair, improvements in the sensory apparatus, and a brain necessarily 

 more highly developed in connection with the possession of improved 

 sensory and motor mechanisms. Having, then, the advantages of 

 superior speed and agility and the capacity for a wider range of favor- 

 able reactions to external conditions, these early mammals survived 

 through a long period which was the "golden age" for reptiles but a 

 time of extraordinary peril for other vertebrates. Apparently the 

 mammals did little more than merely survive. They remained obscure 

 and small animals, most of them less than a foot long. No doubt their 

 chief concern was to keep out of the way of their gigantic but clumsy 

 reptilian contemporaries, and in this their smallness, speed, agility, 

 and better brains must have served them well. Even so, it is likely that 

 they suffered much from the carnivorous reptiles. Fossil Mesozoic 

 mammals are very rare. It may well be that a fossil carnivorous reptile 

 accounts for many mammals which escaped fossilization because they 

 had been eaten by the reptile. 



Toward the close of the Mesozoic, the scene begins to change. 

 Reptiles are on their way out. After long ages of mere survival in a 

 w orld dominated by reptilian monsters, the mammals, hitherto obscure 

 and "meek," begin to "inherit the earth." The incoming of the Ceno- 

 zoic Age finds the mammals well started on their career as a Class, 

 becoming larger and stronger and undergoing a diversification destined 

 to give them world dominance and make the Cenozoic an Age of 

 Mammals. 



The evolutionary differentiation of mammals, in its main lines, 

 repeats that of reptiles. Beginning as small, obscure, indifferently 

 specialized land animals, the group expanded and became highly 



