834 DISTRIBUTION AND AETIOLOGY 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 

 w^ould 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 Ameiican 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 

 obviousl}^ 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 Idotliea and an Asiatic form 

 in the Arctic ocean at the present day. 



