96 REGENERATION 



tail of a lizard again, which is very liable to injury, becomes regen- 

 erated because, as we have seen, it is of great importance to the indi- 

 vidual and if lost its owner is placed at a disadvantage." And as an 

 example of vagueness, the following statement commends itself : 

 " Finally the complexity of the individual parts constitutes the third 

 factor which is concerned in regulating the regenerative power of the 

 part in question ; for the more complex the structure is, the longer 

 and the more energetically the process of selection must act in order 

 to provide the mechanism of regeneration, which consists in the 

 equipment of a large number of different kinds of cells with the sup- 

 plementary determinants which are accurately graduated and regu- 

 lated as regards their power of multiplication." 



Without attempting to disentangle the ideas that are involved in 

 these sentences, let us rather attempt to get a general conception of 

 Weismann's views. In a later paper (1900), in reply to certain criti- 

 cisms, he has stated his position somewhat more lucidly. In the 

 following statement I have tried to give the essential part of his 

 hypotheses : Weismann believes the process of regeneration to be 

 regulated by " natural selection " ; in fact, he states that it has arisen 

 through such a process in the lower animals since they are more 

 subject to injury and that it has been lost in the higher forms 

 except where, on account of injury, it has been retained in certain 

 parts. Thus when Weismann speaks of regeneration as being an 

 adaptation of the organism to its environment, we must understand 

 him to mean that this adaptation is the result of the action of natural 

 selection. We should be on our guard not to be misled by the state- 

 ment that because regeneration is useful to the animal, it has been 

 acquired by natural selection, since it is possible that regeneration 

 might be more or less useful without in any way involving the idea 

 that natural selection is the originator of this or of any other adapta- 

 tion. It will be seen, therefore, that in order to meet Weismann on 

 his own ground it will be necessary to have a clear understanding in 

 regard to the relation of regeneration to Darwin's principle of natural 

 selection. With Weismann's special hypothesis of the " mechanism," 

 so-called, by which regeneration is made possible we have here noth- 

 ing to do, but may consider it on its own merits in another chapter. 



In order to have before us the material for a discussion of the 

 possible influence of natural selection on regeneration, let us first 

 examine the facts that bear on the question of the liability of the 

 parts to injury and their power to regenerate, and in this connection 

 the questions concerning the renewal of parts that are thrown off by 

 the animals themselves in response to an external stimulus are worthy 

 of careful consideration. A comparison between the regeneration of 

 these parts with that of other parts of the same animal gives also 



