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 4 
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- 
tium and A. nobilis 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 
