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THE AMERICAN NATURALIST [Vol. XLIX 



The list of mechanisms, organs and tissues of the animal 

 body and of specific organic or inorganic substances 

 within the body, whose conditions and concentration re- 

 main relatively constant, could be extended. The descrip- 

 tion of these phenomena bulks large in the extensive lit- 

 erature of physiology of to-day. Sufficient data have been 

 adduced to show that there is a considerable basis of fact 

 for the interpretations which we now wish to present. 

 A summary of the known facts of the internal conditions 

 of the organisms permits of the statement that in the 

 homoiothermal animals, there are, then, several mech- 

 anisms of extreme delicacy and great constancy under 

 similar conditions and varying but little under wide 

 changes of external conditions. The tracing out of their 

 development constitutes a large part of the subject matter 

 of comparative physiology. But their interpretation 

 rather than their development is the thing of main in- 

 terest at present. We will return to the questions of the 

 origin and development of these mechanisms in later 

 papers. 



What point of view will give us the best insight into 

 the role of these mechanisms in the evolution of the verte- 

 brate phylum? Of what use have they been? Or are 

 they simply mechanisms which have arisen in the course 

 of evolution apparently through correlation with other 

 phases of development but without obvious significance 

 to the organism? 



III. The Laws of Chemical Equilibrium 

 Before attempting the interpretation of these mech- 

 anisms, or pointing out their role in evolution, we may 

 very briefly review the laws of chemical equilibrium as 

 exemplified in the ' ' slow" reactions of the physical chem- 

 ist. For our present purpose, these laws may be included 

 under the law of mass action, Van't Hoff's law and the 

 phase rule. A fuller statement is given in Blackman's 

 paper and in the text-books of physical chemistry. 



