95 THE AMERICAN NATURALIST [VOL. XL VII 



in the nature of many of these responses ; but it is quite 

 otherwise, if we consider the organisms as wholes. It 

 is clear that all organisms have not only the power of 

 reacting to an external change, but many of their reac- 

 tions are adaptive to a surprising degree. This is indeed 

 the very crux of the difference between living organisms 

 and lifeless things. A lifeless thing can not adjust its 

 internal to its external relations so that it can continue 

 to exist in a changed environment. A crystal in a solu- 

 tion of its kind must dissolve, if the concentration is kept 

 ever so little below saturation; a whole universe must 

 pass away, if anywhere within it there is a persistent 

 uncompensated difference of potential. With living things 

 it is quite otherwise. They have the power of interposing 

 resistances to the potential difference. All living things 

 without exception have adaptive responses so that they 

 are able to continue in existence even though their sur- 

 roundings change in many different ways. They possess 

 adaptability. Their responses due to their irritability 

 are adaptive responses. The irritability of the organism 

 as a whole is, then, above everything else characterized 

 by power of adaptive response. 



It is not difficult to imagine how this specialization of 

 the general property of irritability arose. Some of the 

 indefinite responses of the original organisms to environ- 

 mental change protected the organism against the change. 

 Organisms with such responses survived and their de- 

 scendants had the property of a limited adaptive response 

 to this particular change. From this crude beginning 

 further progress was easy. The changes in the environ- 

 ment, though many, are not indefinite in number, and 

 adaptations in the nature of direct responses easily arose 

 and were perfected. 



Adaptability, then, appears to the physiologist as the 

 master word of evolution. And many facts also may be 

 urged as confirming this conclusion. For example, one 

 and all of the great physiological mechanisms of the body 

 have a single purpose: to secure adaptability. Not to 



