DISTRIBUTION 23 



isthmus cdiTesponds with that on the other. A close 

 agreement would dispose him to consider that not so very 

 long ago, in the large measurements of geological time, 

 the Atlantic and Pacific may have been connected by a 

 natural canal. From great divergence, such as is known 

 to exist in the molluscan fauna, he would infer that the 

 passage existed, if ever, only at a very remote period. In 

 the large freshwater lakes of Southern Sweden, it was dis- 

 covered Fome thirty years back, that a remarkable marine 

 fauna existed, and the inference ingeniously drawn from a 

 review of all the connected facts has been, that these sheets 

 of water were at one time part of the sea, but have been 

 cut off from it by the gradual elevation of the land. Upon 

 this supposition, w^hile the water was gradually losing its 

 saltness, its marine inhabitants, with equal steps, were 

 becoming habituated to a freshwater existence. But it 

 must not be forgotten that the transfer of marine animals to 

 brackish and fresh waters may take place by various modes 

 of migration quite independently of geological changes. It 

 has been noticed as curious, that shells, insects, and plauts, 

 inhabiting fresh water, are of comparatively few species, 

 but those few very widely distributed. Mr. Belt ingeni- 

 ously remarks that, in the oscillations of sea and land, the 

 oceanic and continental domains, though shifting, are con- 

 tinuous, whereas every freshwater area is liable to be again 

 and again completely overwhelmed. By this means the 

 freshwater species of narrow range may be entirely de- 

 stroyed, and only families of wide distribution will survive. 

 The application of this theory to the Crustacea is worthy 

 of study, but the facts which it is designed to explain do 

 not embrace the whole of the globe. The Isopod Asellus 

 aquaticus and the Amphipod Gammarus pulex are obvious 

 instances of freshwater species with an enormous range. 

 Yet from the fresh waters of the Malay archipelago the 

 Asellidas and Gammaridge are said to be entirely wanting. 

 On the other hand, Professor Max Weber has recently 

 observed that, while Europe can show but seven species of 

 freshwater decapods, the Indian archipelago can boast of 

 more than eighty. 



