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. 
