DISTRIBUTION OF GENERA AND SPECIES. IQQ 



Italy and Sicily it comprises two-thirds of all the terres- 

 trial species, while in Sweden and Scotland, and in 

 Switzerland, where elevation produces the effect of a 

 high latitude, its species are less than half of the whole 

 number. The only countries on the list, that have been 

 thoroughly mvestigated, are Great Britain and France. 

 The disproportionate number of species shown to belong 

 to the latter, is due in part to its thorough exploration, 

 but in a greater degree to its extent, which reaches from 

 the Mediterranean to the North Sea, and to the singular 

 variety of its surface, which, although in general by no 

 means mountainous, yet includes parts of the mountain 

 ranges of the Pyi^enees, the Jura, and the Alps, and 

 therefore partakes of a greater diversity of climate and 

 elevation than any other country of Europe. The effect 

 of these peculiarities upon its fauna is very apparent, 

 the northern species, except Clausilia, all the southern 

 species, and all those affecting mountainous regions, 

 being in excess. The value of tables of this kind is 

 very evident from this example. A complete series of 

 them, including especially those of limited districts 

 which present strongly-marked topographical or climatal 

 peculiarities, would be invaluable in affording the means 

 of deducing the laws of geographical distribution. 



When we commenced our introductory remarks, we 

 did not expect to extend this division of our subject 

 beyond a comparison between our species and those of 

 Europe ; but, an opportunity having occurred of exam- 

 imng the valuable work of M. De la Sagra, on the 



VOL. I. 21 



