334 DISTRIBUTION AND ATIOLOGY OF THE CRAYFISHES, 
latter and the West American Astaci; and the closer 
resemblance between the latter and the Pontocaspian 
crayfish, than either bear to the Amur-Japanese form. 
If the facts had been the other way, and the West 
American and Amur-Japanese crayfish had changed 
places, the case would have been intelligible enough. 
The primitive Potamobine stock might then have been 
supposed to have differentiated itself into a western 
astacoid, and an eastern cambaroid form;* the latter 
would have ascended the American, and the former the 
Asiatic rivers. As the matter stands, I do not see that 
any plausible explanation can be offered without recourse 
to suppositions respecting a former more direct com- 
munication between the mouth of the Amur, and that 
of the North American rivers, in favour of which no 
definite evidence can be offered at present. 
The most important negative fact which remains to 
be accounted for is the absence of crayfishes in the 
rivers of a large moiety of the continental lands, and in 
numerous islands. Differences of climatal conditions are 
obviously inadequate to account for the absence of cray- 
fishes in Jamaica, when they are present in Cuba; for 
their absence in Mozambique, and the islands of Johanna 
and Mauritius, when they are present in Madagascar ; 
and-for their absence in the Nile, when they exist in 
Guatemala. 
* Just as there is an American form of Jdothea and an Asiatic form 
in the Arctic ocean at the present day. 
