272 THE ORIGIN OF THE NERVOUS SYSTEM 



cases. Such procedure is of value in two ways: first, it 

 indicates to what extent the theory or working hypothesis 

 may serve; and, second, the consideration of the various 

 questions from a particular viewpoint may provide a 

 basis for further investigation, which will confirm the 

 suggestions or show them to be incorrect. 



Questions of detail aside, the general conception on 

 which the book is based involves three main theses, 

 which are, I believe, of fundamental importance for 

 biological theory. These theses are: first, that devel- 

 opment is a physiologically continuous process; second, 

 that the problem of organismic pattern conceived in 

 any other terms than those of relation to the external 

 world becomes essentially a metaphysical rather than a 

 scientific problem and it is necessary to postulate an 

 entelechy, a soul, or some other metaphysical ordering 

 and integrating factor; third, that in the excitatory 

 relation between protoplasm and the external world 

 and the effects of such excitation on protoplasm we 

 have an adequate physiological basis for organismic 

 pattern and for the physiological continuity of develop- 

 ment. If the book contributes toward the establishment 

 of these three theses as statements of fundamental bio- 

 logical fact, it will have attained its chief purpose. 



