CHAP. XIV.] 



ST. HELENA. 



291 



tlie same direction, so that any transmission of insects by tlieir 

 means must almost certainly be from South Africa. Now there 

 is undoubtedly a South African element in the insect-fauna, but 

 there is no less clearly a European, or at least a north-temperate 

 element, and this is very difficult to account for by causes now 

 in action. But when we consider that this northern element is 

 chiefly represented by remote generic affinity, and has therefore 

 all the signs of great antiquity, we find a possible means of 

 accounting for it. We have seen that during early Tertiary 

 times an almost tropical climate extended far into the northern 

 hemisphere, and a temperate climate to the Arctic regions. 

 But if at this time (as is not improbable) the Antarctic regions 

 were as much ice-clad as they are now, it is certain that an 

 enormous change must have been produced in the winds. In- 

 stead of a great difference of temperature between each pole 

 and the equator, the difference would be mainly between one 

 hemisphere and the other, and this might so disturb the trade 

 winds as to bring St. Helena within the south temperate region 

 of storms — a position corresponding to that of the Azores and 

 Madeira in the North Atlantic, and thus subject it to violent 

 gales from all points of the compass. At this remote epoch the 

 mountains of equatorial Africa may have been more extensive 

 than they are noAV, and may have served as intermediate stations 

 by which some northern insects may have migrated to the 

 southern hemisphere. 



We must remember also, that these peculiar forms are said 

 to be northern only because their nearest allies are now found 

 in the North Atlantic islands and Southern Europe ; but it is 

 not at all improbable that they are really widespread Miocene 

 types, which have been preserved mainly in favourable insular 

 stations. They may therefore have originally reached St. 

 Helena from Southern Africa, or from some of the Atlantic 

 islands, and may have been conveyed by oceanic currents as 

 well as by winds.^ This is the more probable, as a large 



1 On Petermann's map of Africa, in the new edition of Stieler's Hand- 

 Atlas (1879), the Island of Ascension is shown as seated on a much larger 

 and shallower submarine bank than St. Helena. The 1,000 fathom line 

 round Ascension encloses an oval space 170 miles long by 70 wide, and 



