CONNECTING AND MISSING LINKS 



mals is shown by their complex jaw, which consisted of sev- 

 eral bones on either side, and by their retention of a quadrate 

 bone, as it is called, between the jaw and skull. Both of 

 these features had been possessed by reptilian, amphibian, 

 and fish ancestors as far back as we can trace the bony skele- 

 ton, and are thus a firmly established heritage of the race. 

 Why the simplifying.^ And where shall we find the missing 

 bones? These questions have given rise to much argument. 

 At all events, the changes occurred concomitantly with the 

 assumption of other diagnostic features, and in a compara- 

 tively short time. The Triassic rocks yield jaws of creatures 

 so near the dividing line between reptile and mammal that 

 only the most intensive modern research has decided their 

 status as reptiles, although for years they were considered 

 primitive mammalian forms. Mammal and bird each have 

 warm blood, which means not only a heat-controlling 

 mechanism, but clothing (hair or feathers) as well. Feathers 

 are seen in Archaeornis, but fossil hair is still unknown in 

 association with ancient mammals, so that in them warm 

 bloodedness cannot be proved, though it may be assumed 

 from analogy. From the mammal-like reptile sprang the rep- 

 tile-like mammal, out of which true mammals in turn arose. 

 Much of this history is recorded in hundreds of tiny jaws 

 and teeth recovered from rocks of the reptilian age. Certain 

 links in the chain are missing, but the main evolutionary 

 lines are indicated by tangible evidence, not alone by infer- 

 ence. 



Mammalian divergence has followed several lines, all 

 traceable to a few parent stocks. These lines led to the 

 hoofed cohort, to the clawed carnivores, and to those groups 

 which went down to the sea and became adapted mar- 

 vellously to life in the great waters — whales and sea cows 

 of diverse lineage. Other feebler folk became the inhab- 



[263] 



