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
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 incomprehensibly 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
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 reason to believe
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
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