PREFACE 
Concerning the embryonic development and the 
evolution of nervous structure a large body of fact and 
hypothesis is at hand, but the problem of the origin of the 
nervous system as an organ of excitation, transmission, 
and integration has received much less attention. Irri- 
tability, that is, excitability, is commonly regarded as a 
fundamental property of living protoplasm, and it is 
often asserted that the nervous system does not repre- 
sent a new function, added at a certain stage of develop- 
ment, or of evolution, to the original functional complex 
of the organism, but rather a development of the primi- 
tive mechanisms of excitation and transmission. Not 
infrequently, however, particularly within recent years, 
it has been maintained that the primary factors in 
physiological correlation and integration consist of 
chemical substances and their transportation by one 
means or another. These two conceptions are difficult 
to reconcile, for the one implies that excitation and 
transmission are primary factors in physiological corre- 
lation, the other that they are of secondary origin. As 
a matter of fact, we know that excitation and transmis- 
sion occur in protoplasm in the absence of anything which 
we can identify as nervous structure; moreover, it is 
difficult to conceive a living organism incapable of at 
least some degree of excitation and transmission. 
Considered from a physiological viewpoint, the origin 
of the nervous system must be sought in conditions 
present before the appearance of definitely recognizable 
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