ering the vast reduction of brachiopods, and
the fact that our existing cephalopods, though few in number, are more
highly organised than their ancient representatives. 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. Under this point of view, the productions of Great Britain
stand much higher in the scale than those of New Zealand. Yet the
most skilful naturalist, from an examination of the species of the two
countries, could not have foreseen this result.

Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a correspondin