88 THE ORIGIN OF THE NERVOUS SYSTEM 
or regeneration, etc., along the chief body axis. In 
fact, evidence of one kind or another for the existence 
of gradients of some sort as characteristic features of 
axiate organisms appears in the literature of many 
different and widely separated fields of investigation, 
both physiological and morphological. This evidence 
is perhaps the more valuable because the authors were 
in most cases stating or describing facts rather than 
attempting to formulate theories. In the conception 
of the excitation gradient and its fixation in protoplasm 
I have merely attempted to provide a general physio- 
logical basis for the facts as they stand. 
