IN MODERN SCIENCE. 



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suppose that a plant originating in the long arc- 

 tic summers of a warm period might, on migrat- 

 ing southward into the alternations of day and 

 night, undergo material changes. A marine 

 animal long confined to a limited sea-basin 

 might, on being permitted to expand over a 

 wide submerged continent, be greatly modified 

 in its structure and habits. Up to a certain 

 point we know that such changes have oc- 

 curred, and Barrande himself has largely illus- 

 trated them. As an example which I have my- 

 self studied, I may refer to the common shells 

 known on our coasts as sand-clams (My a trun- 

 cata and Mya arenaria). The former species, 

 in the cold waters of the Glacial Age, assumed 

 a short form which it still retains in the arctic 

 regions, and occasionally in the colder waters 

 of the more temperate regions, though there a 

 more elongated form prevails. Evidently the 

 two forms are interchangeable according to the 

 temperature of the water. Still, if we could 

 imagine a permanent refrigeration over all the 

 area pccupied by the animal, the short form 

 only might survive, and might be supposed to 

 be a distinct species. This did not occur, how- 

 ever, even in the Glacial Age, and is not likely 

 to occur. Further, the allied, though quite dis- 



