THE ANIMAL ANCESTRY OF MAN Jl 



As long as the animal crawls with the belly near the 

 ground, the lurching, sinuous movements are pronounced, 

 but by the time of the higher mammal-like reptiles of the 

 Triassic period a notable advance toward the mammalian 

 mode of locomotion had been achieved, in that the body 

 was beginning to be raised further from the ground and the 

 feet to be drawn in toward the mid-line. Meanwhile several 

 of the ribs in the sacral region of the backbone became 

 widened out at the farther ends and attached by ligaments 

 to the pelvis, which thus began to assume even greater 

 importance in the mechanism of locomotion. 



OUR ANCESTORS BECOME WARM-BLOODED 



At this point let us turn aside from the consideration of 

 the more conspicuous parts of the locomotor apparatus in 

 order to trace the internal improvements that were prereq- 

 uisite for its fmal development. In the lower vertebrates, 

 including the fishes and reptiles, the body temperature is 

 both relatively low and relatively variable, so that the 

 animals are not able to maintain their own body temperature 

 and vital activities at a high level during severe changes in 

 the surrounding medium. The mammals are able to do this, 

 not only because their red corpuscles, being both far more 

 numerous and smaller than those of the lower vertebrates, 

 effect a quicker and larger consumption of oxygen in a 

 given time, but also because they have more efficient lungs 

 and a special bellows-like organ, the diaphragm, which 

 acts in a way like a forced draught; while its piston-like 

 action, described by Sir Arthur Keith, no doubt accelerates 

 the circulation and consequent metabolism. Moreover the 

 body is covered with hair, which encloses a layer of non- 

 conducting air, and the skin is full of sweat glands and oil 

 glands, which further assist in the regulation of the body 

 temperature; in addition to these are the complete separation 

 of the venous and arterial blood in the heart and several 

 other details leading to more rapid aeration of the blood 

 and a greater liberation of energy. 



The higher physiological status of mammals is also shown 

 in their improved methods of reproduction. Whereas with 

 few exceptions reptiles lay large eggs, well stored with yolk. 



