DISTRIBUTION 23 
isthmus corresponds 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 some 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, while 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 plants, 
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 pulew are obvious 
instances of freshwater species with an enormous range. 
Yet from the fresh waters of the Malay archipelago the 
Asellidee and Gammaride 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. 
