THE DISTRIBUTION OF CRABS AND CRAYFISHES. 337 



and fluviatile crabs (Thelphusa) compete for the posses- 

 sion of the freshwaters ; and it is not improbable that 

 under some circumstances, they may be more than a 

 match for crayfishes; so that the latter might either be 

 driven out of territory they already occupied, as Astacus 

 leptodactylus is driving out A. nobilis in the Russian 

 rivers; or might be prevented from entering rivers already 

 tenanted by their rivals. 



In connection with this speculation, it is worthy of 

 remark that the area occupied by the fluviatile crabs is 

 very nearly the same as that zone of the earth's surface, 

 from which crayfish are excluded, or in which they are 

 scanty. That is to say, they are found in the hotter 

 parts of the eastern side of the two Americas, the West 

 Indies, Africa, Madagascar, Southern Italy, Turkey and 

 Greece, Hindostan, Burmah, China, Japan, and the 

 Sandwich Islands. The large-clawed fluviatile prawns 

 are found in the same regions of America, on both 

 east and west coasts, in Africa, Southern Asia, the 

 Moluccas, and the Philippine Islands ; while the Atyida 

 not only cover the same area, but reach Japan, extend 

 over Polynesia, to the Sandwich Islands, on the north, 

 and New Zealand, on the south, and are found on both 

 shores of the Mediterranean ; a blind form (Troglocaris 

 Schniidtii), in the Adelsberg caves, representing the blind 

 Cambarus of the caves of Kentucky. 



The hypothesis respecting the origin of crayfishes 



23 



