and may set in motion a whole series of organic revolutions
before it is restored. A change of climate in any direction will be sure to
injure some and benefit other species. The one will consequently diminish,
the other increase in number; and the former may even become extinct. But
the extinction of a species will certainly affect other species which it
either preyed upon, or competed with, or served for food; while the
increase of any one animal may soon lead to the extinction of some other to
which it was inimical. These changes will in their turn bring other
changes; and before an equilibrium is again established, the proportions,
ranges, and numbers, of the species inhabiting the country may be
materially altered. The complex manner in which animals are related to each
other is well exhibited by the importance of insects, which in many parts
of the world limit the numbers or determine the very existence of some of
the higher animals. Mr. Darwin says:--"Perhaps Paraguay offers the most
curious instance of this; for here neither cattle, nor horses, nor dogs
have ever run wild, though they swarm southward and northward in a wild
state; and Azara and Rengger have shown that this is caused by the greater
number in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as
they are, must be {116} habitually checked by some means, probably by other
parasitic insects. Hence, if certain insectivorous birds were to decrease
in Paraguay, the parasitic insects would probably increase; and this would
lessen the number of navel-frequenting flies--then cattle and horses would
run wild; and this would certainly alter (as indeed I have observed in
parts of South America) the vegetation: this again would largely affect the
insects, and this, as we have seen in Staffordshire, the insectivorous
birds, and so onwards in ever increasing circles of complexity."

Geographical changes would be still more important, and it is almost
impossible to exaggerate the modifications of the organic world that might
result from them. A subsidence of land separating a large island from a
continent would affect the animals and plants in a variety of ways. It
would at once modify the climate, and so produce a series of changes from
this cause alone; but more important would be its effect by isolating small
groups of individuals of many species and thus altering their relations t