DISTRIBUTION OF ANIMALS. 



227 



Riitimeyer, in his treatise already cited. In what follows 

 we may essentially adhere to the latter. 



Our knowledge of the regions of distribution of the 

 animal world is still extraordinarily deficient. What do 

 we know, for instance, of the occurrence of marine ani- 

 mals? Few years only have elapsed since the depths of 

 the sea were rendered accessible to research, and the 

 result has almost entirely upset our earlier notions of 

 the geological significance of the sea-bottom and its 

 habitability. After the strong impulse given by Maury 

 to the investigation of the physical condition of the sea, 

 we are now occupied in ascertaining the submarine tem- 

 peratures and currents, the constitution of the sea-bottom, 

 the occurrence of deep-sea organisms, and the conditions 

 of their existence. We are therefore just beginning to 

 collect the material for a future geography of marine 

 organisms. Among terrestrial animals, certain groups 

 of which the actual distribution can be defined, are use- 

 less for our general purpose. 



Butterflies, for instance, which are an easy prey to 

 currents of air, defy geological barriers, and, above all, 

 that impor,tant partition which from the tertiary era has 

 been erected, or rather excavated in the bottom of the 

 sea, between Australia and India.'''- It is the same with 

 bats, and also with migratory, predatory, and aquatic 

 birds; while, as Wallace shows, the other orders of this 

 class are in tropical regions very reliable and stable in- 

 habitants of their often Hmited districts, seemingly sug- 

 gestive of migration. Exclusive of these, there remains 

 therefore little more than the Mammalia, whose extrac- 

 tion may be inferred with certainty from a comparison 

 of their present cantonments (Cantonirung), — an expres- 



