90 HOMOIOTHERMISM 



individual marine animal as for that of one on land, 

 where the environmental conditions varied so greatly. 

 That terrestrial animals have achieved the ability to 

 maintain a rather uniform nutrient level is exemplified 

 in the constancy of the composition of their blood, even 

 in extreme environmental fluctuations. The blood of the 

 higher forms has become of constant reaction. Thus 

 fluctuations in the hydrogen-ion concentration of the en- 

 vironment do not noticeably affect that of the pericel- 

 lular media. This is accomplished by the presence of 

 '^buffers" in the blood and the coordinated activity of 

 the respiratory center and the kidney. 



The more complex animals have also attained a re- 

 sistance to, or independence of, variations in the salinity 

 and osmotic pressure in the environment. In the sea, 

 with its miiformity of salt content, there was little need 

 for independence. The lower Metazoa contain almost 

 exactly the same proportion and concentration of salts 

 as the sea water which they inhabit. In the higher 

 Metazoa the salinity and osmotic pressure of the blood 

 may differ greatly from that of the surrounding medium. 

 Internal constancy is maintained by the selective elimi- 

 nation of certain constituents by the kidneys. Macal- 

 lum (1904) maintained that the saline composition of the 

 blood of higher vertebrates closely resembles the prob- 

 able composition of sea water at the time when the proto- 

 vertebrate types first acquired a kidney. The latest stage 

 through which certain animals have passed in becoming 

 independent of environmental fluctuations through the 

 attainment of a constant internal environment has been 

 the maintenance of constant body temperature. In 

 aquatic habitats, especially in the sea, the variations in 

 temperatures are slight as compared to those in the air. 

 Furthermore the range of variation in water is usually 



