CHAPTER X 



GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER 

 MOLLUSCA — THE PALAEARCTIC, ORIENTAL, AND AUSTRAL- 

 ASIAN REGIONS 



The Mollusca afford specially valuable evidence on prob- 

 lems of geographical distribution. This fact is largely due to 

 their extreme susceptibility to any change in the conditions of 

 life. Genera which are accustomed to live in a certain temper- 

 ature and on certain food, cannot sustain life if the temperature 

 falls or rises beyond certain limits, or if the required food be 

 not forthcoming. There is therefore a marked contrast between 

 the Mollusca of the tropics and of the temperate zones, while 

 different regions in the same latitude, whether within or with- 

 out the tropics, often show great diversity in their fauna. 

 Every region is thus characterised by its Mollusca. The Mol- 

 lusca, for instance, of Australia or of South Afiica characterise 

 those countries quite as much as do the kangaroo and the emu, 

 the hartebeest and the ostrich ; there is nothing like them any- 

 where else in the world. In the Greater Antilles the Mollusca 

 stand out beyond all other forms of life as characteristic of the 

 islands as a whole, and of each separate island in particular. 



The geographical distribution of the land and fresh-water 

 Mollusca must be considered quite apart from that of the marine 

 Mollusca. The sea offers no such serious barriers to the spread 

 of the latter as the land does to the spread of the former. If we 

 were to journey to the Azores, and turn our attention to the 

 land-snails, we should find them almost wholly peculiar, while 

 amongst the sea-shells we should recognise many as occurring 

 also on our southern coasts, and few that were different from 

 those of the Mediterranean. The marine Mollusca of the Sand- 



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