DISTRIBUTION OF GENERA AND SPECLES. K39 
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 investigated, 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 Pyrenees, the Jura, and the Alps, and 
therefore partakes of a greater diversity of climate and 
elevation than any other country of Em-ope. 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- 
ining the valuable work of M. De la Sagra. on the 
vol. 1. 44 
