86 THE LIFE OF CRUSTACEA 



case of the Barnacles, which are fixed to one spot 

 when adult, a locomotive larval stage is clearly a 

 necessity. But, here as elsewhere, to demonstrate 

 the usefulness of any character is to go only a very 

 little way towards explaining its origin. Moreover, 

 the mere necessity for a locomotive larva throws no 

 light on the remarkable resemblances between the 

 larval stages of widely different species. In the 

 adult state, a Branchiopod, a Copepod, an Ostra- 

 cod, a Barnacle, and a Penaeid Prawn, are separated 

 by enormous differences of form and structure ; yet, 

 as we have seen, all these are hatched from the egg 

 as six-limbed nauplius larvse differing from each 

 other only in trivial details. It seems hardly possible 

 to imagine any other interpretation of this very 

 striking fact than is afforded by the theory of Evolu- 

 tion. We are forced to assume that all these diverse 

 forms of Crustacea are descended from very similar 

 or identical ancestral types, and that the modifica- 

 tions arising in the course of their evolution have 

 affected the adult but not the larval stages. Some 

 naturalists would go farther than this, and would 

 apply the so-called "theory of recapitulation" to 

 the larval stages of the Crustacea. According to 

 this theory, the stages in the development of any 

 animal tend to recapitulate, more or less closely, the 

 history of the race. Thus it is assumed, for instance, 

 that the nauplius reproduces the structure of a six- 

 limbed ancestral form, from which, in the distant 



