chap, x.] THE PAL^EARCTIC REGION. 209 



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. 



Colcoptera. — 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, 212 species 

 have been obtained, of which 1 75 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 



