98 



THE AMERICAN NATURALIST [Vol. XLVII 



mammals are rendered superior to the attacks of many of 

 their enemies and to this extent rendered superior to their 

 environments. 



Third, there is the mechanism for rendering mammals 

 tolerably independent of the moisture content of their 

 environment, a mechanism most highly developed in the 

 reptiles. A mechanism formed by the replacing of the 

 wet skin of the amphibian by a dry or scaly skin; the 

 perfecting of the kidneys to maintain osmotic pressure 

 of the blood ; the control of the sweat glands and loss of 

 water by the intestines ; the development of membranes 

 non-permeable to salts, so that animals may sit in fresh 

 water and not lose their salts. One of the most interest- 

 ing parts of this mechanism is shown in the reptiles and 

 birds, in the substitution of uric acid for urea in their 

 excretions. By this improvement reptiles have secured 

 almost complete independence of the water content of 

 their environments. They make enough water in their 

 own bodies to supply their small losses. This again is 

 a mechanism of which we can trace the steady growth 

 without a break from the invertebrates to man. 



A fourth great mechanism makes mammals independent 

 of barometric fluctuations and less dependent on a fixed 

 atmosphere. By means of their blood loaded with hemo- 

 globin carried in corpuscles lacking all oxygen-consuming 

 power, they are able to live on lofty plateaus, or in deep 

 valleys; and in the presence of much or little carbon 

 dioxide. 



The mechanisms having to do with reproduction and 

 the caring for the young afterward have this same 

 advantage of rendering the mammals independent of 

 environment. 



A sixth mechanism is the alimentary mechanism, most 

 highly perfected in man. This has rendered him inde- 

 pendent of any particular kind of food. He can make 

 his body of any kind of plant or animal. He can make 

 carbohydrate out of protein and many other things. He 

 can live in any climate largely because of this mechanism. 



