CHAP. X.] THE PALiEARCTIG REGION. 201* 
although these comprise many fresh species, the two faunas and 
that of the continent still remain almost as distinct from each 
other as before. It has been already stated (p. 31) that the 
means by which land mollusca have been carried across arms 
of the sea are unknown, although several modes may be 
suggested ; but it is evidently a rare event, requiring some 
concurrence of favourable conditions not always present. The 
diversity and specialization of the forms of these animals is, 
therefore, easily explained by the fact, that, once introduced 
they* have been left to multiply under the influence of a variety 
of local conditions, which inevitably lead, in the course of ages, 
to the formation of new varieties and new species. 
Coleoptera . — The beetles of Madeira and the Canaries have 
been so carefully collected and examined by Mr. T. V. Wollaston, 
and those of the Azores described and compared by Mr. Crotch, 
and they illustrate so many curious points in geographical dis- 
tribution, that it is necessary to give some account of them. No 
less than 1,480 species of beetles have been obtained from the 
Canaries and Madeira, only 360 of which are European, the 
remainder being peculiar to the islands. The Canaries are 
inhabited by a little over 1,000 species, Madeira by about 700, 
while 240 are common to both ; but it is believed that many of 
these have been introduced by man. In the Azores, 21 2 species 
have been obtained, of which 175 are European ; showing, as in 
the birds, a closer resemblance to the European fauna than in 
the other islands which, although nearer to the continent, offer 
more shelter and are situated in a less tempestuous zone. Of 
the non* European species in the Azores, 19 are found also in 
the other groups of islands, 14 are peculiar, while 3 are American. 
Of the European species, 132 are found also in the other Atlantic 
islands, while 43 have reached the Azores only. This is inter- 
esting as showing to how great an extent the same insects 
reach all the islands, notwithstanding the difference of latitude 
and position ; and it becomes of great theoretical importance, 
when we find how many extensive families and genera are alto- 
gether absent. 
The Madeira group has been more thoroughly explored than 
r 
