inevitably exerts on the climate.<16> We see, then, that the
presence of these different animals can be explained only by
supposing great secular changes in climate. Let us see if we can
strengthen this view by an appeal to the vegetation of
this period.

We have seen how important a guide as to climate were the
remains of the vegetation of the early times. We therefore turn
with more confidence to such discoveries as will tell us of the
flora of this age. But there are many reasons why remains of
plant growth should be few. As we shall soon learn, this was a
period of flooded rivers; and in the gravels and loams thus
formed is found our principal source of information as to the
life of the age. But such a rush of waters would form gravelly
banks or great beds of loam, and would sweep any plants which
might be washed into its floods far out to sea; or if by chance
they should become buried in such gravel beds, the action of
water would speedily cause the decay of the tender portions,
such as leaves, bark, and soft wood, in which case no profitable
investigation could be made. Occasionally, however, around the
shores of old lakes, vegetable beds have been buried, and we
know that some mineral springs deposit a sort of protecting
sediment on every thing with which they come in contact. By such
means, at rare intervals, leaves, seeds, and fruits have been
sealed up for future inspection, and from a careful study of all
such instances much valuable information has been obtained.
At one place in the valley of the Seine was discovered, under a
bed of tufa, the remains of a forest growth. It is not doubted
that the deposit belongs to the Glacial Age.<17>