No. 627] 



ADAPTATION 



343 



seem to be closely related to that of Jennings, and in- 

 deed Jennings himself views it in this light. It is diffi- 

 cult to gather, however, to what extent Holmes has in 

 mind the principle of "trial and error." His compari- 

 son of regeneration with functional hypertrophy does 

 not seem compatible with this principle. ''Eemove one 

 of a pair of organs," he says, "and its fellow increases 

 in size. Eemove a part of one of these organs and the 

 remaining portion grows, forms new tissue, and regen- 

 erates the missing part. ' ' Furthermore, he believes that 

 these phenomena may be analogous with some of those 

 described under the name of "chemical equilibrium." 



The decomposition of compounds in solution proceeds until there is a 

 definite relation established between the amounts of the old compounds 

 and the new. If the chemical equilibrium thus established is disturbed 

 by the removal of one of these compounds more of that compound will 

 be produced; and the more rapidly the compound is removed, the more 

 rapidly it is formed. 



Such an "automatic" restoration of equilibrium as 

 this might seem to be a radically different thing from 

 trial and error. The process by which it is attained 

 would appear to be direct and unhesitating. Holmes 

 says that the solar system, no less than the organism, is 

 a ' ' self -regulating mechanism. ' ' Now, in the former, the 

 balance of its opposing forces is effected "automati- 

 cally" in the sense that any deviation in the movement 

 of one of the parts would result inevitably in a compen- 

 sating deviation in the others. Is the restoration of an 

 organism to its norm of this direct and automatic type? 

 Are such processes as tend to compensate a disturbance 

 in the normal functioning of an organism the direct and 

 exclusive result of the disturbance itself, or does this dis- 

 turbance evoke a variety of responses of which the suit- 

 able response may finally happen to be one? The first 

 of these alternatives may be admitted as probable in the 

 case of such disturbing factors as have been frequently 

 experienced in the past. But how does it happen that 

 certain cells of the iris of a newt become stimulated to 

 division by the removal of the lens? And why should 



