No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 327 



as these interpretations and inferences have to do with 

 the general problem of evolution. We may, then, first of 

 all briefly review the more salient of these facts, and after- 

 ward attempt their interpretation in terms of well-known 

 physico-chemical laws — e. g., the law of mass action and 

 the phase rule. 



1. Thermo-Regulation in the Higher Organisms 

 We may divide the vertebrates into two general groups 

 on the basis of their internal or body temperature; (1) 

 those which maintain a relatively constant internal tem- 

 perature and (2) those whose internal temperature is 

 variable. These groups may be tabulated as follows: 12 



Animals with 



the temperature falls! 

 below 20° C. 



Looking now to the differences between lower and higher 

 vertebrates, to restrict ourselves at the start to a rela- 

 tively small part of the animal kingdom, one of the most 

 noticeable changes has been the development of a very 

 constant body temperature in the so-called warm-blooded 

 or homoiothermal animals (Table II). The detailed 

 enumeration of the body temperature of all the animals 

 so far observed would require too much space, but the fol- 

 lowing data will be sufficient to show upon what basis of 

 fact the statements rest: 



"Richet, Dictionnaire de Physiologie, 1898, t. Ill, pp. 85-86. 



