OUR ANCIENT RELATIVES 



fluids, in the breathing organs. The locomotor 

 machinery was vastly improved, the brain and 

 nervous system had to keep pace with the general 

 advance and a new and much less wasteful method 

 of reproduction had to be perfected. 



Among the heat-regulating devices arising in 

 the mammals, we note the following: (a) the dia- 

 phragm, a complex structure arising from the 

 conjunction of various muscle layers of the neck 

 and abdomen; it acts as a bellows to draw fresh 

 air into the lungs and thus to increase the con- 

 sumption of oxygen; the liberation of heat is a 

 by-product; the glands in the skin multiplied and 

 gave rise to (b) sebaceous glands, pouring out a 

 wax-like substance that tends to keep the skin 

 soft and pliable; (c) sudoriparous or sweat glands, 

 lowering the body temperature by evaporation of 

 the exuded moisture. 



Chief among the heat-retaining structures was 



(d) the hair, which seems to have arisen from small 



tactile outgrowths of the skin. These at first grew 



out between the scales and later supplanted them. 



We do not know exactly when this substitution 



took place, as the skin of soft-skinned animals is 



very rarely, if ever, fossilized, but the later mam- 



41 



