256 ISLAND LIFE 



the globe, and the only rational exi^lanation of such facts 

 is, that they are indications of very ancient and once wide- 

 spread groups, maintaining themselves only in a few 

 widely separated portions of what was at one time or 

 another the area of their distribution. 



Land-shells of the Azores. — Like the insects and birds, 

 the land-shells of these islands have a generally European 

 aspect, but with a larger proportion of peculiar species. 

 This was to be expected, because the means by which 

 molluscs are carried over the sea are far less numerous and 

 varied than in the case of insects ; ^ and we may therefore 

 conclude that their introduction is a very rare event, and 

 that a species once arrived remains for long periods un- 

 disturbed by new arrivals, and is therefore more likely to 

 become modified by the new conditions, and then fixed as 

 a distinct type. Out of the sixty-nine known species, 

 thirty-seven are common to Europe or the other Atlantic 

 islands, while thirty-two are peculiar, though almost all 

 are distinctly allied to European types. The majority of 

 these shells, especially the peculiar forms, are very small, 

 and many of them may date back to beyond the glacial 

 epoch. The eggs of these would be exceedingly minute, 

 and might occasionally be carried on leaves or other 

 materials during gales of exceptional violence and duration, 

 while others might be conveyed with the earth that often 

 sticks to the feet of birds. There are also, probably, other 

 unknown means of conveyance ; but however this may be, 

 the general character of the land-molluscs is such as to 

 confirm the conclusions we have arrived at from a study of 

 the birds and insects, — that these islands have never been 

 connected with a continent, and have been peopled with 

 living things by such forms only as in some way or other 

 have been able to reach them across many hundred miles 

 of ocean. 



The Flora of the Azores. — The flowering-plants of the 

 Azores have been studied by one of our first botanists, Mr. 

 H. C. Watson, who has himself visited the islands and 

 made extensive collections ; and he has given a complete 

 catalogue of the species in Mr. Godman's volume. As our 

 1 See Chap. V. p. 78. 



