ON DYNAMIC INFLUENCES IN EVOLUTION. 3 



suits are concerned, for the ground to strike the horse's hoof 

 would be the same as for the horse to strike the ground with 

 his hoof ; direction and dynamic value of shock being as 

 sumed to be equal in the two cases. Since individual organ 

 isms usually appear free to wander about or remain quiescent, 

 the idea that they are under constant stress does not ordinar 

 ily suggest itself. To this habit of superficial observation I 

 ascribe the slowness with which the dynamic element in evolu 

 tion has received recognition, though pointed out clearly so 

 long ago, by Herbert Spencer. 



That which distinguishes the organic individual from the in 

 organic fragment of matter is the complexity of its reaction 

 to these impacts, which reaction we term physiological in con- 

 tra-distinction to the simply mechanical, though both, at bot 

 tom are doubtless similar. 



The characters which develop in an organism in response to 

 these impacts are acquired, but that which is transmitted 

 is a facility of response in the same line, which may, under 

 favorable circumstances, lead to a similar response in the 

 progeny, and, in the course of time w,ith a continuation of 

 similar impacts through successive generations, promote and 

 establish the physiological habit which is the directive influ 

 ence toward the regular development of the characters in 

 question. 



It is, I believe, generally admitted that such is the case in 

 relation to mental stimuli and reactions in man and some of 

 the higher animals and that the growth of intellectual life in 

 the human race depends upon it. 



It is a matter of indifference, dynamically, whether the par 

 ticular series of impacts concerned in developing a special 

 physiological response is the result of conscious effort by the 

 organism or not ; but, as it is highly unlikely that any volun- 



