of life tend to supplant the old and unimproved
forms.

By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the varieties of the same species at the present {282} day; and
these parent-species, now generally extinct, have in their turn been
similarly connected with more ancient species; and so on backwards, always
converging to the common ancestor of each great class. So that the number
of intermediate and transitional links, between all living and extinct
species, must have been inconceivably great. But assuredly, if this theory
be true, such have lived upon this earth.

_On the lapse of Time._--Independently of our not finding fossil remains of
such infinitely numerous connecting links, it may be objected, that time
will not have sufficed for so great an amount of organic change, all
changes having been effected very slowly through natural selection. It is
hardly possible for me even to recall to the reader, who may not be a
practical geologist, the facts leading the mind feebly to comprehend the
lapse of time. He who can read Sir Charles Lyell's grand work on the
Principles of Geology, which the future historian will recognise as having
produced a revolution in natural science, yet does not admit how
incomprehensively vast have been the past periods of time, may at once
close this volume. Not that it suffices to study the Principles of Geology,
or to read special treatises by different observers on separate formations,
and to mark how each author attempts to give an inadequate idea of the
duration of each formation or even each stratum. A man must for years
examine for himself great piles of superimposed strata, and watch the sea
at work grinding down old rocks and making fresh sediment, before he can
hope to comprehend anything of the lapse of time, the monuments of which we
see around us.

It is good to wander along lines of sea-coast, when formed of moderately
hard rocks, and mark the {283} process of degradation. The tides in most
cases reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there is
good evidence that pure water can effect little or nothing in wearing away
rock. At last the base of the cliff is undermined, huge fragments fall
down, and these remaining fixed, have to be worn away, atom by atom, until
reduced