Characters, Hereditary and Acquired. 77 



constructed, still more must it have been the case 

 with regard to all the previous stages of construction. 

 For here, without elaborating the point, it would 

 appear that a process of construction by survival of 

 the fittest alone is incomprehensible. 



On the other hand, of course, the theory of use- 

 inheritance furnishes a fully intelligible whether or not 

 a true explanation. For those nerve-centres in the 

 spinal cord which co-ordinate the muscles required for 

 retracting the feet are the centres used by the will 

 for this purpose. And, by hypothesis, the frequent 

 use of them for this purpose under circumstances 

 of stimulation which render the muscular response 

 appropriate, will eventually establish an organic 

 connexion between such response and the kind of 

 stimulation to which it is appropriate even though 

 there be no utilitarian reason for its establish- 

 ment l . To invert a phrase of Aristotle, we do not 

 frequently use this mechanism because we have it 

 (seeing that in our normal condition there is no 

 necessity for such use) ; but, by hypothesis, we have 

 it because we have frequently used its several elements 

 in appropriate combination. 



I will adduce but one further example in illustra- 

 tion of these general principles passing at once 

 from the foregoing case of comparative simplicity 

 to one of extreme complexity. 



There is a well-known experiment on a brainless 

 frog, which reveals a beautiful reflex mechanism in 



1 It may be said, with regard to this particular reflex, that it may 

 perhaps be, so to speak, a mechanical accident, arising from the 

 contiguity of the sensory and motor roots in the cord. But as this 

 suggestion cannot apply to other reflexes presently to be adduced, it need 

 not be considered. 



