122 How Animals Changed 



account of a deficiency in the environment, and often in aquatic and 

 terrestrial habitats where an impervious external covering of a large 

 body make the carrying of sufficient oxygen to internal tissues 

 difficult. 



Body fluids changed, as animals were gradually transformed from 

 marine to fresh water and land animals, from actual sea water to 

 comparatively dilute, exactly regulated, viscous, nutritious, corpus- 

 culated, and pigmented liquids. Instead of furnishing merely suit- 

 able media for bathing living cells, they have come to serve a variety 

 of functions concerned with nutrition, respiration, excretion, the 

 humoral control of activities, and the prevention of damage from 

 disease or injury. The history of the rise of land animals has been 

 a bloody one. Emphasis has shifted from dependence on a primitive 

 dependable external medium to that on a specialized, highly regu- 

 lated, internal medium. 



Metabolism 



The quality of metabolism in all animals is apparently similar, 

 but the rate varies markedly on account of inherent hereditary 

 qualities and because of variations in environment. Size has a 

 marked influence on rate of metabolism on account of surface-mass 

 relations. A man to keep his body warm must expend food equal 

 to about 1% of his body weight each day; a mouse, 25%; and an 

 insect, if it were to maintain the same temperature as a mammal, 

 625% (Kennedy, 1927a). It is apparent that an insect must con- 

 tinue to be poikilothermic, and can be active therefore only when 

 its environment is warm. Animals consume about the same amount 

 of total energy per gram during their lives (Rubner, 1924b). 

 Smaller animals live faster, shorter lives than larger animals. Rats 

 live twice as long if prevented from exercising as when they exercise, 

 because they expend energy more slowly (Slonaker, 1926) . The 

 longevity of many poikilothermic animals is more or less directly 



