224 THE ORIGIN OF THE NERVOUS SYSTEM 



physiological condition as regards their capacity for exci- 

 tation. If, however, the conduction path is a prede- 

 termined gradient the maximal excitation may differ 

 quantitatively at different levels of the gradient, that is, 

 a gradient in the quantitative characteristics of maximal 

 excitation may occur along the path. Moreover, we 

 might expect to find a rise in threshold with progress 

 down the gradient. Such a path, even though the 

 reaction is strictly "all or none" at all levels, may con- 

 duct with a decrement. If it is long enough, a limit 

 may even be reached, beyond which conduction cannot 

 proceed because the stimulus resulting from maximal 

 excitation at this point is so weak that it falls below the 

 threshold for points beyond. The "all-or-none" type 

 of reaction does not then exclude the possibility of con- 

 duction with a decrement and of a limited range or 

 distance of conduction. In such a path, however, the 

 steepness of the decrement and the position of the limit 

 are determined by the path, not by the quantitative 

 characteristics of the original stimulus. 



As pointed out in chapter iv, the primitive forms of 

 transmission do not show the u all-or-none' : reaction, 

 but quantitatively different stimuli determine quanti- 

 tatively different reactions between certain limits. A 

 slight stimulus may produce a slight response only in 

 the region immediately about the point of stimulation, 

 a stronger stimulus a stronger response, which is trans- 

 mitted to a greater distance, and so on up to the maxi- 

 mum. Even in more or less specialized conduction 

 paths like the ctenophore plate row such differences 

 appear and are different in degree according to the 

 direction of conduction in the gradient. Even in the 



