32 "THE COMING OF MAN 



would have fouled the water. It slowly became a lung. In 

 these marshy pools overgrown during the wet season with 

 all sorts of weeds and plant-life, the paddle-like fins would 

 have been of little service to forms hovering or crawling along 

 the bottom; they became jointed and changed into legs, and 

 gradually the amphibious animal emerged more and more and 

 ventured out on the land. 



Much still remained to be done before the body was fully 

 shaped in all its parts. The blood-system had to be enlarged 

 and modified in connection with air-breathing. The lungs 

 became more complex and spongy, offering a larger respiratory 

 surface. In the thick growth of ferns and club-mosses the 

 eyes were for a time of less use than the sense of smell, and 

 the higher development of this sense reacted on the brain. 

 The skeleton of a form supported by four legs, rather than 

 floating or swimming in the water, gained increased strength 

 and stiffness. In backbone and skull and elsewhere cartilage 

 was replaced by bone under the stimulus of new strains. 

 The muscular system of shoulder and thigh became heavier 

 and more complex, that of the trunk began to lighten. 



Perhaps the most important change of all was the rise of 

 temperature in the body. In the better conducting medium 

 of the water the heat produced within escaped rapidly and 

 was lost. The temperature of the body of a fish can be only 

 slightly higher than that of the surrounding water and varies 

 accordingly. Heat radiates into air far less rapidly, hence 

 the temperature of the body gradually rises. It is still 

 variable in amphibia and modern reptiles, in mammals and 

 birds it will become constant and high. 



All chemical changes go on more rapidly as temperature 

 rises, but all organs are not equally affected. The more un- 

 stable the material of the tissue, the greater the quickening 

 of the changes. Cartilage and bone are slightly stimulated, 

 muscle far more; the markedly protoplasmic structures like 

 nerve-centers and glands w^ill be stimulated most of all, and 

 their development will be hastened proportionately. 



Life on land brought new difficulties and dangers but also 



