WHAT EVOLUTION IS 115 



natural selection is a process that is in 

 actual operation in the world about us. 



Natural selection, however, is not 

 without its limitations. It is an op- 

 eration that at best can lift organ- 

 isms only to the level of positive needs. 

 Nature, with a certain prodigality, 

 often goes much beyond this limit. 

 Examples are abundant enough. 

 Many crustaceans have the curious 

 habit of casting injured legs and 

 other appendages. When a crab loses 

 a part of one of its legs, it recovers 

 by a new growth, but this new growth 

 does not replace simply the lost part; 

 the old stump is thrown off from a 

 so-called casting joint at the base of 

 the leg and a wholly new leg is 

 formed. Most crabs, picked up on 

 the shore at random, are undergoing 

 this process on some one of their nu- 

 merous appendages. 



Hermit crabs live with the pos- 



