ii6 WHAT EVOLUTION IS 



terior parts of their bodies tucked 

 away in some dead snail-shell appro- 

 priated by the crab for this purpose. 

 Their hind appendages are therefore 

 protected and, according to Morgan, 

 who recorded this case, these append- 

 ages are never found suffering from 

 injury as their front legs are. Never- 

 theless, if by experimental steps the 

 hind appendages are injured, they are 

 cast and recovered as the exposed 

 appendages are. Here then is an in- 

 stance where nature has stepped be- 

 yond the actually necessary, and 

 where it would be difficult to offer for 

 the condition an explanation based 

 purely upon natural selection. 



Another instance of the same kind 

 occurs among certain almost micro- 

 scopic crustaceans, the copepods. 

 These small creatures are abundantly 

 represented in the surface waters of the 

 sea. Copepods of different sexes are, as 



