instance, Reptiles, at the period when the earth was not fully redeemed
from the waste of waters, and extensive marshes afforded means for the
half-aquatic, half-terrestrial life even now characteristic of all our
larger Reptiles, while Insects, so dependent on vegetable growth, make
their appearance with the first forests; so that we need not infer,
because these and other classes come in after the earlier ones, that
they are therefore a growth out of them, since it is altogether probable
that they would not be created till the conditions necessary for their
maintenance on earth were established. From a merely speculative point
of view it seems to me natural to suppose that the physical and the
organic world have progressed together, and that there is a direct
relation between the successive creations and the condition of the earth
at the time of those creations. We know that all the beings of the
Silurian and Devonian periods were marine; the land, so far as it
existed in their time, was a great beach, and along those shores,
wherever any part of the continents was lifted above the level of the
waters, the Silurian and Devonian animals lived. Later, in the marshes
and the fern-forests of the Carboniferous period, Reptiles and Insects
found their place; and only when the earth was more extensive, when
marshes had become dry land, when islands had united to form continents,
when mountain-chains had been thrown up to make the inequalities of the
surface, were the larger quadrupeds introduced, to whose mode of
existence all these circumstances are important accessories.

But while all the types and most of the classes were introduced upon the
earth simultaneously at the beginning, these types and classes have
nevertheless been represented in every great geological period by
different sets or species of animals. In this sense, then, there has
been a gradation in time among animals, and every successive epoch of
the world's physical history has had its characteristic population. We
have found that there is a correspondence between the gradation of
structural complication among adult animals as known to us to-day, which
we may call the Series of Rank, and the gradation of embryological
changes in the same animals, which we may call the Series of Growth; and
there is also a correspondence between these two series and the order of
succession in time, that establishes a certain gradation in the
introduction of animals upon eart