102 THE DARWINIAN THEORY 



may be effected. Hence such larvae may take 

 weeks, months, or even years in recapitulating these 

 later stages, which their larger-egged allies get 

 through in a few days. Moreover, during the whole 

 of this time they are exposed to competition, both 

 amongst themselves and with other animals ; they 

 have to obtain food for themselves, and they are 

 liable to be themselves devoured as food by other 

 animals. 



Owing to this competition, and to the length of 

 time during which it lasts, these larvcz are liable to 

 acquire, through natural selection, characters which 

 are connected with their existence as larvce, but which 

 form no part of the ancestral history ; characters 

 which will aid them in obtaining food, or in escaping 

 from their enemies, but which were not found in any 

 of the ancestors of the species. Of such secondary 

 larval characters the long spines with which the 

 Pluteus larva of sea-urchins are provided are good 

 examples ; so also are the enormous spines on the 

 young larvae of crabs and other Crustacea. Other 

 excellent illustrations are afforded by the develop- 

 mental history of many fresh-water forms, in which, 

 from the danger of their being swept down by the 

 currents of the rivers or streams in which they 

 dwell, or to obtain protection from the cold of winter, 

 special characters are often acquired. The glochidium 

 larva of the fresh-water mussel is a good instance of 

 the former, and the specially-protected statoblasts or 

 winter buds of Polyzoa and sponges, of the latter 

 specially-acquired character. 



It is not easy to distinguish between these later 



