336 THE NATURALIST IN NICARAGUA. [Ch. XVIII. 



still preserved in a few of the large rivers of the 

 world. 



To illustrate more clearly the theory I have advanced, 

 I will take a supposititious case. In the southern states of 

 America there is reason to suppose that since the glacial 

 period there has been a great variation in the species 

 of the fresh- water mollusk genus Me lama, and in dif- 

 ferent rivers there are distinct groups of species. Now 

 let us suppose that the glacial period was to return, and 

 that the icy covering, gradually thickening in the north, 

 should push down southward as it did once before. The 

 great lakes of North America would he again filled with 

 ice, and every one of their inhabitants destroyed. As 

 the ice advanced southward, the inhabitants of one river- 

 system after another would be annihilated, and many 

 groups of Melania entirely destroyed. On the retreat of 

 the ice again the rivers and lakes would reappear, but 

 the varieties of animals that had been developed in them 

 would not, and their places would be taken by aquatic 

 forms from other areas, so that the number of species 

 would be thereby greatly reduced, and wide- spreading 

 forms would be freed from the competition of many 

 improved varieties. 



Viewed in this light, the similarity of fresh -water pro- 

 ductions all over the world, instead of being a difficulty 

 in the way of the acceptance of the theory of natural 

 selection, becomes a strong argument in favour of its 

 truth ; for we perceive that the number of marine, terres- 

 trial, and fresh-water animals is in proportion to the 

 more or less continuous development that was possible 

 under the different conditions under which they lived. 



The same line of argument might be used to explain 



