100 HOMOIOTHERMISM 



must attain stability, but, equally, he must avoid 

 monotony. 



Conclusions. — Man today proudly dominates the 

 earth. He has attained to his present position through 

 a long struggle up from ancestors in the dim past. Gill- 

 breathers in the water attained some ability to live on 

 land, developed lungs, and took up terrestrial life. At 

 first land vertebrates lacked proper insulation. Doubt- 

 less many of them lost the water from their bodies and 

 died. But as time went on tough skins developed which 

 held water in. Later these skins grew specialized feath- 

 ers and hairs which served to conserve bodily heat as 

 well as water. Then constant temperatures at optimum 

 levels for metabolism became possible and were devel- 

 oped through various controlling mechanisms. Given 

 a constant bodily temperature, which permitted peren- 

 nial activity, and a highly efficient brain, man has been 

 able to excel all competitors by inventions which make 

 him more independent of environment than any other 

 animal. Yet, he is not, and probably never will be, 

 wholly emancipated. He must still be more or less 

 ** natural.'' 



