326 THE AMERICAN NATURALIST [Vol. XLIX 



elude the origin within the marine organism of stereo- 

 chemical isomers, the great importance of which has re- 

 cently been pointed out by Reichert. 11 Stability of con- 

 ditions would even favor the perpetuation of such com- 

 pounds as could be formed under a given set of condi- 

 tions. (2) On the land, as has already been indicated, the 

 higher animals— birds and mammals— have acquired a 

 relative independence of the environment as shown by 

 their wide distribution. Their internal conditions of tem- 

 perature, moisture and the like, may not only differ 

 greatly from the same conditions in the environment at 

 any given time, but the internal conditions, as we shall 

 show, do not change greatly when the external conditions 

 change. Intermediate between these two types is a third 

 type which lives in an environment subject to wide varia- 

 tions of temperature and moisture, and whose internal 

 temperature varies with the change of external tempera- 

 ture. While some of the internal conditions of these or- 

 ganisms remain relatively stable, others are subject to wide 

 variation. We have then to explain, in terms of function, 

 (1) what are the various mechanisms by which the higher 

 animals have attained this relative independence of the 

 environment and (2) what has been the role of these 

 mechanisms in organic evolution. 



II. The General Constancy of Internal Conditions in 

 the Higher Organisms 

 The study of the internal physico-chemical relationships 

 of higher animals has led to the acquisition of a consid- 

 erable mass of facts concerning this phase of animal or- 

 ganization. These facts show that in the higher animals 

 there exist a number of mechanisms which interact to 

 bring about a remarkable constancy of internal physico- 

 chemical conditions during the life of the animal. It ap- 

 pears justifiable again to call attention to some of these 

 facts with a brief description of some of the mechanisms 

 involved in order better to emphasize some of the inter- 

 pretations of, and inferences drawn from, them in so far 



" Reichert, Science, 1914, N. S., XI, pp. 649-661. 



