CHAP. xxn. IN BRITAIN AND NORTH AMERICA. 435 



late Mr. Kirby, have always agreed to regard as mere 

 varieties of tlie same species. 



Mr. T. V. Wollaston, in treating of the variation of insects 

 in maritime situations and small islands, has shown how the 

 colour, growth of the wings, and many other characters, 

 undergo modification under the influence of local conditions, 

 continued for long periods of time ; * and Mr. Brown has lately 

 called our attention to the fact, that the insects of the Shet 

 land Isles present slight deviations from the corresponding 

 types occurring in Great Britain, but far less marked than 

 those which distinguish the American from the European 

 varieties.! In the case of Shetland, Mr. Brown remarks, a 

 land communication may well be supposed to have prevailed 

 with Scotland at a more modern era than that between 

 Europe and America. In fact, we have seen that Shetland 

 can hardly fail to have Been united with Scotland after the 

 commencement of the glacial period (see map, p. 279); 

 whereas a communication between the north of Europe by 

 Iceland and Greenland (which as before stated, once enjoyed 

 a genial climate), must have been anterior to the glacial 

 epoch. A much larger isolation, and the impossibility of 

 varieties formed in the two separated areas crossing with each 

 other, would account, according to Mr. Darwin's theory, for 

 the much wider divergence observed in the specific types of 

 the two regions. 



The reader will remember that at the commencement of the 

 Glacial Period there was scarcely any appreciable difference 

 between the molluscous fauna and that now living. When 

 therefore the events of the Glacial Period, as described in the 

 earlier part of this volume are duly pondered on, and when we 

 reflect that in the Upper Miocene period the living species of 

 mollusca constitute only one third of the whole fauna, we see 



* Wollaston, On the Variation of f Transactions of Northern Entomo- 

 Species,&c. London, Van Voorst, 1856. logical Society, 1862. 



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