34 INDIVIDUALITY IN ORGANISMS 



continued or frequently repeated excitation is the estab- 

 lishment of a gradient in the condition of the protoplasm 

 which constitutes a more or less permanent material 

 substratum for a persistent metabolic gradient inde- 

 pendent of the local external stimulus. In short the 

 effect of the local action of an external factor on 

 protoplasm may sooner or later result in the estab- 

 lishment of a metabolic gradient, or the material basis 

 for such a gradient, which persists for a longer or shorter 

 time after the external factor has ceased to act. As a 

 matter of fact, such gradients, once established, often 

 persist throughout the life of the individual. These 

 gradients may be directly visible in a graded structure of 

 the protoplasm as well as in differences in rate of reaction, 

 or they may appear only or chiefly in the differences in 

 rate, according to the nature of the protoplasm. There 

 is considerable evidence to show that when once estab- 

 lished to a certain degree they tend to persist and even to 

 become more marked, because the rate and extent of 

 further changes in the protoplasm at different levels 

 of the gradient are determined by the differences in rate 

 of reaction at these different levels. As a rapidly flow- 

 ing stream quickly removes from its channel obstacles 

 which a slowly flowing stream removes only slowly or 

 not at all, so the changes in protoplasm which make a 

 higher rate of reaction possible are more rapid and more 

 extensive with a high than with a low rate of reaction. 

 If these considerations are correct, and there are, 

 as will appear, many facts to support them, it is evi- 

 dent that a persistent metabolic gradient associated 

 with a material gradient in the protoplasmic sub- 

 stratum may arise as the result of the local or differential 



