CHAPTER X 



GEOGKAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOL- 



LUSCA THE PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN 



REGIONS 



The Mollusca afford specially valuable evidence on problems 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 temperature 

 and on certain food, cannot sustain life if the temperatm^e falls or 

 rises beyond certain limits, or if the required food be not forth- 

 coming. 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 without the 

 tropics, often show great diversity in their fauna. Every region 

 is thus characterised by its Mollusca. The Mollusca, for instance, 

 of Australia or of South Africa characterise those cou.ntries quite 

 as much as do the kangaroo and the emu, the hartebeest and the 

 ostrich ; there is nothing like them anywhere 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 

 IMollusca. 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 occmTing 

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

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



