258 THE ORIGIN OF THE NERVOUS SYSTEM 
being effectively transmitted to greater distances down 
than up a gradient. If the electric current produced 
at a region of excitation is the transmitting factor this 
difference is readily understood (p. 97). As structural 
differentiation of transmission paths progresses, the 
morphological features of this relation to the gradients 
become more and more evident, until the conditions 
found in the higher animals and man are attained. 
Moreover, as regards the functional relations of neurons, 
even if we admit that conduction may occur Without 
decrement in the single axon, the probability exists of 
quantitative differences in excitation in different neu- 
rons and of the relation of these differences to levels of 
the gradients and to the changes in the gradients which 
occur during development and evolution. 
These suggestions do not involve the assumption 
that the growth of the axons is determined by the 
passage of nervous impulses, but if the suggestions 
advanced in chapters iv and xi have any basis in fact, 
the growth of the axon and the transmission of excitation 
are both determined by the electrical factor. The 
growth of the axon is the physiological effect of increase 
in internal positivity produced by the electric currents 
in a physiological gradient, and transmission is the 
physiological effect of the electrical currents associated 
with a local excitation, this effect being primarily, 
according to Lillie, an increase in internal positivity. 
From this viewpoint the outgrowth of the axon and 
the transmission of excitation are simply reactions of 
protoplasm to essentially the same electrical conditions, 
and it is not strange that they should exhibit certain 
similarities. 
