492 C. M. Child 



that they become essentially one-sided rather than mutual. If 

 the restoration of a part like that removed were the exception 

 rather than the rule, or even if it were less frequent, we might still 

 accept the hypothesis. But a hypothesis which can account for 

 the typical phenomena within its field, only with the aid of addi- 

 tional special assumptions, which in this case amount practically 

 to throwing over the hypothesis, can scarcely be regarded as satis- 

 factory. 



The numerous cases already known where an animal is capable 

 of replacing a part repeatedly after successive removals seem to 

 me to furnish additional evidence against Holmes' theory. Even 

 so important a part as the head may be replaced repeatedly in 

 many forms without appreciable change in character. It is 

 scarcely probable, to say the least, that the relations between the 

 head-region and other parts are one-sided in the sense that it is 

 dependent on them., while they are independent of it. And if the 

 relation is not one-sided in this sense, we should expect that 

 repeated removals of the head would bring about essential changes 

 in the other parts, even if the first removal did not. If such 

 changes in the other parts do occur to any marked extent, it is 

 difficult to understand how a new head like the old can be replaced 

 time after time as the result of "social pressure," for such changes 

 in the old parts must alter the character of the "social pressure." 

 Here then Holmes' theory leads us into something closely approach- 

 ing a dilemma. 



Holmes continues: "If the parts B-F w^re more plastic, 

 absence of A would naturally tend to cause greater changes in 

 them, especially if new tissues were not produced in place of yf , 

 which would come to assume some of the missing functions before 

 the modification extended very far. There would then be a pro- 

 gressive modification extending from the region of y^, which would 

 tend to become less the farther it extended, but eventually perhaps 

 affecting more or less the entire organism. Functional equilib- 

 rium would then be maintained by working over the organism 

 so that all the parts were adjusted to functioning on a smaller 

 scale. The different methods of regulation, through morphal- 

 laxis, regeneration and the various combinations of these proc- 



