No. 554] ADAPTATION AND THE PHYSIOLOGIST 96 



adapt an organism to one environment, but to all environ- 

 ments, and thus to make it superior to all environments. 

 Furthermore, the higher organisms are specially remark- 

 able for the development of that master tissue of the body 

 which is preeminently irritable and of which the main 

 function is the adjustment of internal to external rela- 

 tions, the nervous system ; and finally that the inference 

 is sound may be concluded from the fact that it is by 

 adaptability and by no other quality whatever that organ- 

 isms may be arranged in the order of their evolutionary 

 progress. 



It is not at all surprising that adaptability should be 

 the most important adaptation in nature, overpowering, 

 except in special cases, and dominating all others. For 

 there is but one certain thing in nature: namely uncer- 

 tainty. The most constant feature of all environments, 

 but particularly of land environments, has been their in- 

 constancy. Changeableness is the chief characteristic of 

 all environments, whatever their special characters may 

 be. There are changes of light, temperature, climate, 

 oxygen and carbon dioxide, moisture; changes due to 

 the introduction of new species by migration upsetting 

 nature 's balance ; changes in the food supply. Climates, 

 flora and fauna change; change alone persists. Change 

 is the essential thing. We may expect, therefore, if 

 Darwin be correct in his conclusion that variation and 

 natural selection account for evolution, that adaptation 

 to changeableness must be the chief adaptation in nature, 

 and more than all others, it must have determined the 

 general course of evolution. This is found to be the case 

 and the great physiological mechanisms of the body are 

 designed, as already stated, to subserve this fundamental 

 adaptation. Adaptability is that power which fits organ- 

 isms to withstand the unexpected : the vicissitudes of life ; 

 special adaptations of form and color may contribute to 

 the survival of animals ; but the essential, or root, adapta- 

 tion is to changeableness. By adaptation to all environ- 

 ments they become finally superior to all environments. 



