380 Evolution and Adaptation 



by crossing with the uninjured individuals. But it is not 

 necessary to consider this possibility, since there is another 

 fact that shows at once that the power to regenerate could 

 not have been gained through selection. The number of un- 

 injured individuals in each generation will be much greater 

 than the injured ones, and these will have so great an ad- 

 vantage over the injured individuals that, if competition 

 approached the degree assumed by the selectionists, the in- 

 jured individuals should be exterminated. A slight ad- 

 vantage gained through better powers of regeneration would 

 be of little avail in competition, as compared with the com- 

 petition with the uninjured individuals. Since selection is 

 powerless to accomplish its end without competition, and 

 since with competition all the injured individuals would be 

 eliminated, it is clear that an appeal cannot be made to 

 selection to explain the power of regeneration. 



In many cases the power of regeneration could not have 

 been slowly acquired through selection, since the interme- 

 diate steps would be of no use. Unless, for example, a 

 limb regenerated from the beginning almost completely, the 

 result would be of no use to the animal. If the limb did 

 regenerate completely the first time it was injured, then the 

 selection hypothesis becomes superfluous. 



There are also a few cases known in which a process of re- 

 generation takes place that is of no use to the animal. If, for 

 instance, the earthworm {Allolobophora fcetida) be cut in two 

 in the middle, the posterior piece regenerates at its anterior 

 cut end, not a head, but a tail. Not by the widest stretch 

 of the imagination can such a result be accounted for on the 

 selection theory. Again, we find the reverse case, as it were, 

 in certain planarians. If the head of Planaria lugubris is 

 cut off just behind the eyes, there develops at the cut 

 surface of this head-piece another head turned in the opposite 

 direction. Here again we have the regeneration of a per- 

 fect structure, but one that is entirely useless to the in- 



