GEOGRAPHICAL DISTRIBUTION. 349 



Chapter II. 1 



GEOGRAPHICAL DISTRIBUTION OF THE MOLLUSCA. \ 



It is one of the most familiar facts in Natural History, that many countries 

 possess a distinct Fauna and Flora, or" assemblages of animals and plants 

 peculiar to themselves ; and it is equally true, though less generally under- 

 stood, that the sea also has its provinces of animal and vegetable life. 



The most important, or best known of these provinces are indicated on the i 

 accompanying map ; different names, in some instances, and different letters | 

 and numbers being employed to distinguish the marine from the terrestrial 1 

 regions.* i 



The division of the surface of the globe into natural history provinces 

 ought to be framed upon the vsddest possible basis. The geographical ' 

 distribution of every class of animals and plants should be considered, in 

 order to arrive at a theory of universal application. 



The Land Provinces hitherto proposed have been chiefly founded on 

 botanical grounds, but the evidence afforded by insects, and the higher 

 classes of animals, confirms the existence of these divisions. 



The llarine Provinces have also been investigated by botanists ; and the ' 

 striking peculiarities of the fisheries have been taken into account as well as 

 the distribution of shell-fish and corals. j 



In order to constitute a distinct province it is considered necessary that i 

 at least one-half the- species should be peculiar, a rule which applies i 

 equally to plants and animals. Some genera, and sub-genera are limited to 

 each province, but the proportion is different in each class of animals and in i 

 plants, t 1 



Specific areas. — Species vary extremely in their range, some being 



* The author regrets that, on account of the expense, this map appears with- ' 

 out the advantage of colours. He would recommend those who are sufficiently 

 Interested in the subject, to colour their own copies, distinguishing the shores of the 

 marine provinces by the following tints : — 



Blue 1. Arctic province ; 15. Magellanic. i 



Green. 2. Boreal; 11, Aleutian, 5. Aralo-Caspian. 



Orange. 3. Celtic. ' 



Purple. 4. Lusitanian; 10. Japonic; 12. Californian ; 18, Trans-Atlantic. j 



Yellow. 6. W.African; 8. Indo-Pacific; 13. Panamic ; 17. Caribbean. | 



Lake. 7. S.African; 9. Australo-Zealandic; 14. Peruvian; 16. Patagonian. 

 + The genera of plants amount to 20,000, and consist on an average of only 4 

 species apiece! The genera of shells commonly admitted are only 400 in number, 

 and average 40 species each. It follows that the areas of the molluscan genera •' 

 (cceteris paribus) ought to be 10 times as great as those of plants. j 



