320 DISTRIBUTION AND ZTIOLOGY 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 Britain 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 torrentiwm 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- 
