ure, 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 {302} 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 constantly 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 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