320 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



factors : the one, a process of morphological and con- 

 comitant physiological modification ; the other, a process 

 of change in the condition of the earth's surface. 



If we set aside, as not worth serious consideration, the 

 assumption that the Astacus torrentium of Biitain was 

 originally created apart from the Astacus torrentium of 

 the Continent ; it follows, either that this crayfish has 

 passed across the sea by voluntary or involuntary migra- 

 tion ; or that the Astacus torrentium existed before the 

 English Channel, and spread into England while these 

 islands were still continuous with the European main- 

 land ; and that the present isolation of the English cray- 

 fishes from the members of the same species on the 

 Continent is to be accounted for by those changes in the 

 physical geography of western Europe which, as there is 

 abundant evidence to prove, have separated the British 

 Islands from the mainland. 



There is no evidence that our crayfish has been 

 purposely introduced by human agency into Great 

 Britain ; and from the mode of life of crayfish and the 

 manner in which the eggs are carried about by the 

 parent during their development, transport by birds or 

 floating timber would seem to be out of the question. 

 Again, although Astacus nobilis is said to venture into 

 the brackish waters of the Gulf of Finland, and A. lepto- 

 dactylus, as we have seen, makes itself at home in the 

 more or less salt Caspian, there is no reason to believe 

 that Astacus torrentium is capable of existing in sea- 



