We ought also to compare the relative
proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand
kinds of vertebrate animals exist, and if we knew that at some former
period only ten thousand kinds existed, we ought to look at this increase
in number in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organisation of the world. We thus see
how hopelessly difficult it is to compare with perfect fairness, under such
extremely complex relations, the standard of organisation of the
imperfectly-known faunas of successive periods.
We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have seized
on places which must have been previously occupied by the indigenes, we
must believe, that if all the animals and plants of Great Britain were set
free in New Zealand, a multitude of British forms would in the course of
time become thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant
of the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, any considerable number would be enabled to seize on places
now occupied by our native plants and animals.
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