by the waves; while almost all the older
rocks which now form the surface of the earth have been once covered
with newer deposits which have long since disappeared. Nowhere are the
evidences of this denudation more apparent than in North and South
America, where granitic or metamorphic rocks cover an area hardly less
than that of all Europe. The same rocks are largely developed in Central
Africa and Eastern Asia; while, besides those portions that appear
exposed on the surface, areas of unknown extent are buried under strata
which rest on them uncomformably, and could not, therefore, constitute
the original capping under which the whole of these rocks must once have
been deeply buried; because granite can only be formed, and metamorphism
can only go on, deep down in the crust of the earth. What an
overwhelming idea does this give us of the destruction of whole piles
of rock, miles in thickness and covering areas comparable with those of
continents; and how great must have been the loss of the innumerable
fossil forms which those rocks contained! In view of such destruction we
are forced to conclude that our palaeontological collections, rich
though they may appear, are really but small and random samples, giving
no adequate idea of the mighty series of organism which have lived upon
the earth.[183]

Admitting, however, the extreme imperfection of the geological record as
a whole, it may be urged that certain limited portions of it are fairly
complete--as, for example, the various Miocene deposits of India,
Europe, and North America,--and that in these we ought to find many
examples of species and genera linked together by intermediate forms. It
may be replied that in several cases this really occurs; and the reason
why it does not occur more often is, that the theory of evolution
requires that distinct genera should be linked together, not by a direct
passage, but by the descent of both from a common ancestor, which may
have lived in some much earlier age the record of which is either
wanting or very incomplete. An illustration given by Mr. Darwin will
make this more clear to those who have not studied the subject. The
fantail and pouter pigeons are two very distinct and unlike breeds,
which we yet know to have been both derived from the common wild
rock-pigeon. Now, if we had every variety of living pigeon before us, or
even all those which have lived during the present century, we should
find no intermediate types bet