290 



ISLAND LIFE. 



[part II. 



forty-six species have their only (remote) allies in a few insects 

 widely scattered in South Africa, New Zealand, Europe, and the 

 Atlantic Islands. In like manner, eleven species of Bembidium 

 form a group by themselves ; and the Heteromera form two 

 groups, one consisting of three genera and species of Opatridse 

 allied to a type found in Madeira, the other, Anthicodes, alto- 

 gether peculiar. 



Now each of these types may well be descended from a single 

 species which originally reached the island from some other 

 land ; and the great variety of generic and specific forms into 

 which some of them have diverged is an indication, and to 

 some extent a measure, of the remoteness of their origin. The 

 rich insect fauna of Miocene age found in Switzerland consists 

 mostly of genera which still inhabit Europe, with others which 

 now inhabit the Cape of Good Hope or the tropics of Africa and 

 South America ; and it is not at all improbable that the origin of 

 the St. Helena fauna dates back to at least as remote, and not 

 improbably to a still earlier, epoch. But if so, many difficulties 

 in accounting for its origin will disappear. We know that at 

 that time many of the animals and plants of the tropics, of 

 North America, and even of Australia, inhabited Europe ; while 

 during the changes of climate, which, as we have seen, there is 

 good reason to believe periodically occurred, there would be 

 much migration from the temperate zones towards the equator, 

 and the reverse. If, therefore, the nearest ally of any insular 

 group now inhabits a particular country, we are not obliged to 

 suppose that it reached the island from that country, since we 

 know that most groups have ranged in past times over wider 

 areas than they now inhabit. Neither are we limited to the 

 means of transmission across the ocean that now exist, because 

 we know that those means have varied greatly. During 

 such extreme changes of conditions as are implied by glacial 

 periods and by warm polar climates, great alterations of winds 

 and of ocean-currents are inevitable, and these are, as we have 

 already proved, the two great agencies by which the trans- 

 mission of living things to oceanic islands has been brought 

 about. At the present time the south-east trade -winds blow 

 almost constantly at St. Helena, and the ocean-currents flow in 



