314. DISTRIBUTION AND ZTIOLOGY OF THE CRAYFISHES. 
difference are at least as remarkable as the resem- 
blances. 
With respect to the latter, the area oocupied by the 
Potamobiide, corresponds roughly with the Palearctic 
and Nearctic divisions of the great Arctogeal 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 Arctogea. 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 
which have determined the distribution of crayfishes have 
been very different 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 
