94 



Til K A ME L'l ( AX NATURALIST 



[Vol. XLVII 



rally expects to find that the adaptations of function have 

 been of great importance in determining survival. 



Of all the physiological properties of the original proto- 

 plasm upon which natural selection might be supposed to 

 act, irritability, the most fundamental property of living 

 matter, would seem the most probable point of attack; 

 for irritability is that property of protoplasm in virtue 

 of which it adjusts itself to its environment. It is the 

 property of response; and since it is the environment 

 which is acting as the judge of the excellence of the 

 response and doing the selecting, it would seem that it 

 must be upon this property that all organisms must be 

 tested. It is, moreover, this property that Spencer has 

 very acutely selected as the most fundamental charac- 

 teristic of living organisms, namely, the power of continu- 

 ous adjustment of internal to external conditions. It 

 would seem probable that however well animals might be 

 adapted to special environments by the action of natural 

 selection, this particular property, or function, which has 

 to do with the continuous adjustment of internal to ex- 

 ternal relations must have been throughout the whole 

 course of evolution of predominant importance. And if 

 there has been any unity in the progress; if the course of 

 evolution has been at all in any single direction; and if 

 the natural selection theory is true; it must be in the 

 direction of the perfecting of this function. 



I think this short statement will make it clear why the 

 physiologist turns naturally to this fundamental quality, 

 or property of living things, when he considers evolution 

 and adaptation; for however organisms may vary in 

 structure or other particulars, they all have irritability 

 in common. Moreover, I think most physiologists will 

 agree with me that this particular property has been too 

 often neglected by most students of evolution, among 

 whom physiologists have been unfortunately very rare. 



Irritability shows itself in all cells by the power of 

 internal change in response to an external change. In 

 most cells of the body there is nothing especially adaptive 



