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 (AJlolobophora fetida) 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- 
« 
