352 Appendix A 



when we consider that the external forces which awake response 

 in an organism generally belong to its inorganic (physical or 

 chemical) environment, while the usefulness of the response has 

 relation to its organic environment (enemies, prey, etc.). Thus 

 one set of forces supply the stimuli which evoke a response to 

 another and very different set of forces. We can, therefore, 

 accept neither of the suggestions which have been offered. 

 Useful individual modifications are not directly due to the 

 external forces, and are not due to the inherent constitution 

 of the organism. 



" The only remaining hypothesis is that which I have already 

 mentioned, the view that whenever organisms react adaptively 

 under external forces they do so because of special powers 

 conferred on them by natural selection. This hypothesis will, 

 it seems to me, meet and satisfactorily explain all the facts of 

 the case, whether employed as a preparation or as a substitute 

 for hereditary variations accumulated by natural selection." 1 



1 In the Dictionary of Philosophy, art. ' Plasticity,' the present writer points 

 out that the original mobility or plasticity of living matter is probably differ- 

 ent from the more specific plasticity of the developed organisms. J. M. B. 



