204 INDIVIDUALITY IN ORGANISMS 



particular external factor which originally produced 

 them. The reaction of the organism to a sufficient 

 local excitation is not simply a local reaction, but a 

 reaction more or less of the whole organism, and we know 

 that in the case of many physiological reactions the 

 repetition of the reaction in response to repeated external 

 excitation alters the reaction system so that response 

 occurs more readily or more rapidly or with a lower 

 intensity of stimulus. We say that the irritability of the 

 protoplasm is increased, its "threshold" for stimulation 

 is lowered, etc. If this change goes far enough the 

 reaction may occur in the absence of the external factor 

 which first produced it, simply because the condition 

 or constitution of the protoplasm has been so altered 

 by the repetition of the reaction that it occurs auto- 

 matically when any condition determines a sufficiently 

 high metabolic rate in the reaction system. The 

 "inheritance of acquired characters" then belongs in 

 the same general category as the increase in irritability 

 resulting from repeated excitation, but it may in many 

 cases require thousands or hundreds of thousands of 

 generations before a condition approaching auto- 

 maticity in its production is attained. In the face 

 of the physiological facts it is difficult to understand 

 how biologists can continue to maintain the distinction 

 between soma and germ plasm, and to content them- 

 selves with the assertion that natural selection is ade- 

 quate to account for adaptation in the organic world. 

 If the organism is in any sense a dynamic entity, then 

 its evolution must be a reaction determined, on the 

 one hand, by its physico-chemical constitution, and 

 on the other, by its relation with the external world, 



