Rate of Regeneration. 363 



number of removed legs or arms there is an increase and fiot a 

 decrease in the rate of regeneration of each. This striking fact 

 must be reckoned with in any theory bearing on the nature of 

 regeneration. It would be premature to attempt to build up a 

 constructive theory on the basis of the few facts so far discovered. 

 Enough, however, is clear to make profitable the mention of the 

 bearing of the facts on some of the more common theories of 

 regeneration. 



1. Observers who have had to do with the responses of animals 

 to adverse conditions have pointed out again and again the pecu- 

 liar fact that the response to such adverse conditions is in a direc- 

 tion advantageous to the animal. The results of the present ex- 

 periments may therefore be taken as another instance of such a 

 response. The crayfish with the greater number of removed legs 

 and the brittle-star with the greater number of removed arms 

 respond to the greater injury by an increase in the rate of regenera- 

 tion of each member. Obviously the animal with the greater 

 number of removed appendages has more need of the absent organs 

 than does the other. It therefore may seem very plain to some 

 that the rate of regeneration is greater in the one case because 

 there is more need of a rapid replacement in that case. It is but a 

 step further to the familiar statement that the cause of any need 

 in an organism may be taken as a sufficient cause for the fulfill- 

 ment of that need. There are unfortunately a few who have been 

 and will continue to be satisfied with superficial explanations 

 of this character. For these the problem of regeneration is con- 

 sidered solved when the naive statement is made that if a part of 

 an animal is removed it is obviously more advantageous to the 

 animal to regenerate a new part than not to regenerate one. 



2. The suggestion may be made that the animal with the 

 greater number of appendages gone, exercises the regenerating 

 ones more vigorously than does the animal with the smaller num- 

 ber gone. As a result of this greater activity the regenerating 

 appendages grow more rapidly in the former case than in the 

 latter. In support of this idea it may be said, for instance, that 

 the animal which still h^s one uninjured chela concentrates its 

 chela-functions upon that organ, thereby lessening the activity 

 of the regenerating bud. On the other hand the animal with both 

 cheke gone having no uninjured chela upon which to concentrate 

 its chela-functions exercises to their full extent the developing 



