.314 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES. 



difference are at least as remarkable as the resem- 

 blances. 



With respect to the latter, the area oocupied by the 

 PotamohlidcB, corresponds roughly with the Palaearctic 

 and Nearctic divisions of the great Arctogfeal provinces 

 of distribution indicated by mammals and birds; while 

 distinct groups of crayfishes occupy a larger or smaller 

 part of the other, namely, the Austro-Columbian, Aus- 

 tralian, and Novozelanian primary distributional pro- 

 vinces of mammals and birds. Again, the peculiar 

 crayfishes of Madagascar answer to the special features 

 of the rest of the fauna of that island. 



But the North American crayfishes extend much 

 further South than the limits of the Nearctic fauna in 

 general ; while the absence of any group of crayfishes 

 in Africa, or in the rest of the old world, south of the 

 great Asiatic table-land, forms a strong contrast to the 

 general resemblance of the North African and Indian 

 fauna to that of the rest of Arctogaea. Again, there is 

 no such vast difference between the crayfishes of New 

 Zealand, Australia, and South America, as there is 

 between the mammals and the birds of those regions. 



It may be concluded, therefore, that the conditions 

 wliich have determined the distribution of crayfishes have 

 been very diff'erent from those which have governed the 

 distribution of mammals and birds. But if we compare 

 with the distribution of the crayfishes, not that of ter- 

 restrial animals in general, but only that of freshwater 



