PHYSIOLOGICAL SPECIES. 297 



kinds of crayfish under consideration are capable of fertile 

 union or whether they are sterile. It is said, however, 

 that hybrids or mongrels are not met with in the waters 

 which are inhabited by both kinds, and that the breeding 

 season of the stone crayfish begins earlier than that of 

 the noble crayfish. 



M. Carbonnier, who practises crayfish culture on a large 

 scale, gives some interesting facts bearing on this ques- 

 tion in the work already cited. He says that, in the 

 streams of France, there are two very distinct kinds of 

 crayfishes — the red-clawed crayfish (L'Ecrevisse a pieds 

 rouges), and the white-clawed crayfish (L'Ecrevisse a 

 pieds blancs), and that the latter inhabit the swifter 

 streams. In a piece of land converted into a crayfish 

 farm, in which the white-clawed crayfish existed natur- 

 ally in great abundance, 300,000 red-clawed crayfish 

 were introduced in the course of five years ; neverthe- 

 less, at the end of this time, no intermediate forms were 

 to be seen, and the "pieds rouges" exhibited a marked 

 superiority in size over the "pieds blancs." M. Car- 

 bonnier, in fact, says that they were nearly twice as big. 



On the whole, the facts as at present known, seem to 

 incline rather in favour of the conclusion that A. torren- 

 Hum and A. nohilis are distinct species; in the sense 

 that transitional forms have not been clearly made out, 

 and that, possibly, they do not interbreed. 



As I have already remarked, the very numerous 



