35 Problems of ^Organic Adaptation 



phenomena. The development of particular structures and 

 functions fitted to particular conditions of life, such as 

 organs of locomotion, sensation, respiration, digestion, of- 

 fense, and defense, and all the multitudes of diverse forms 

 and ways in which organisms are fitted to carry on the fun- 

 damental properties of life amidst the most varied condi- 

 tions these adaptations we may reasonably expect a theory 

 of evolution to explain, and it is the crowning glory of 

 Darwin's theory that it is, on the whole, able to explain 

 them. 



In the preface to his "Vortrage iiber Descendenz-theorie" 

 Weismann says : "The selection principle controls in fact 

 all categories of life units. It does not create the primary 

 variations, but it does determine the paths of development 

 which these follow from beginning to end, and therewith 

 all differentiations, all advances of organization, and finally 

 the' general course of development of organisms on our 

 earth, for everything in the living world rests on adapta- 

 tion." I have here proposed that the selection principle is 

 also applicable to physiological reactions as well as to vital 

 units, that it lies at the basis of behavior as well as of 

 bodily structure, and that even instinct, intelligence, and 

 purpose are themselves the residuum that is left after the 

 elimination of unfit responses. The selection principle is 

 the only causal and intelligible explanation of all forms of 

 adaptation, and if we reject it we can turn only to non- 

 mechanistic explanations. 



