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ISLAND LIFE 



PART II 



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 transmission of living 

 things to oceanic islands has been brought about. At the 

 present time the south-east trade-winds blow almost con- 

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

 same direction, so that any transmission of insects by 

 their 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 con- 

 sider 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. Instead of a great difference of temperature be- 

 tween 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 now, 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 



