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 
