236 



THE ORIGIN AND EVOLUTION OF LIFE 



of the earliest mammals. Proofs of arboreal habit are seen in 

 the limb-grasping adaptations of the hind foot in many prim- 

 itive mammals, and even in the human infant. Thus the 



Fig. 114. Ancestral Tree of the Mammals. 

 Adaptive radiation of the Mammalia, originating from Triassic cynodont reptiles and 

 dividing into three main branches: (A) the primitive, egg-laying, reptile-like mammals 

 (Monotremes) ; (B) the intermediate pouched, viviparous mammals (Marsupials- 

 opossums, etc.); and (C) the true Placental which branch off from small, primitive, 

 arboreo-insectivorous forms (Trituberculata) of late Triassic time into the four grand 

 divisions (i) the clawed mammals, (2) the Primates, (3) the hoofed mammals, and (4) 

 the cetaceans. Dividing into some thirty orders, this grand evolution and adaptive 

 radiation takes place chiefly during the four million years of Upper Cretaceous and 

 Tertiary time. As among the Reptilia, the primary arboreo-terrestrial adaptive phases 

 radiate by direct evolution into all the habitat zones, and by reversed and alternate evolu- 

 tion develop backward and forward in adaptation to one or another habitat zone. Dia- 

 gram prepared for the author by W. K. Gregory. 



existing tree shrews, the tupaias of Africa (Fig. 112), in many 

 characters resemble the hypothetic ancestral forms of Creta- 

 ceous time from which the primates (monkeys, apes, and man) 

 may have radiated. 



