CHAPTER X 

 THE MAMMALS 



Equaling if not transcending in importance the chemical 

 composition of the internal environment is the regulation 

 of the temperature of that environment to a relatively 

 fixed range, so that, in truth, as Bernard said, the animal 

 lives in a sort of 'hothouse,' protected from the thermal 

 vagaries of the outside world. In the entire animal king- 

 dom this temperature regulation has been achieved 

 twice only, in the birds and in the mammals. 



The time and circimistances of the evolution of warm- 

 bloodedness in either of these classes have not been 

 widely discussed by the paleontologists for the obvious 

 reason that the fossil record affords no direct evidence 

 on the matter. Mammals are insulated against heat loss 

 by means of hair, which is, like feathers, an outgrowth 

 from the epidermis, though wholly different in develop- 

 ment and structure; but imJike feathers, hair is not easily 

 fossilized and it is not known to what extent it was pres- 

 ent in the early forms, which are known only from their 

 jaws, teeth, and skeletal remains. The fossil record with 

 respect to the earliest birds, on the other hand, is limited 

 to two specimens (Archaeopteryx and Archaeornis) 

 from the Jurassic. Consequently we can only speculate 

 as to when either class became warm-blooded. 



Despite many gaps, the evolution of the mammals 

 from the reptihan stem is one of the better known chap- 



