Summary
Darwin addresses the fact that his theory of natural selection
is not supported by findings in the geological (or fossil) record.
If Darwin’s theory of natural selection were true, paleontologists
studying fossils should be able to find intermediate links between
existing species and their parent forms throughout the geological
record. Unfortunately, those intermediate links have rarely been
found. To refute his theory’s critics, Darwin argues that the geological
record is imperfect. Changes in land over time mean that the remains
of all extinct species are not preserved in a manner that has allowed
scientists to reconstruct the history of species development.
Using Charles Lyell’s Principles of Geology,
Darwin argues that the physical makeup of the earth’s surface is
constantly in flux. Gradual land changes can clearly be seen in
the degradation of rocks on the seacoast and the subsequent deposit
of sediment in rivers and ocean, which results in changes to the
geological makeup of both land masses and bodies of water on the
earth’s surface. The rate of these changes, however, is exceedingly
slow—Darwin estimates that the denudation, or erosion, of the Weald,
a forest in England, took more than 300 million
years. The slow pace of changes to the land proves how unimaginably
long the life of the earth has been. Darwin argues that because
the earth has existed for an unfathomable number of years, the number
of changes that have taken place on its surface, including the number
of species that have formed, flourished, and eventually become extinct,
is infinite compared to the paltry holdings of fossils at geological
museums.
Clearly, then, many more species have existed than the
comparatively small number known to scientists. What happened to
all of the other species’ fossil remains? Some fossils may have
decayed in the ocean, as most sediment deposits are not thick enough
to withstand the persistent decay caused by waves. Oscillation of
sea levels may also have played a role in wiping out fossil remains.
New species that formed on land and emerged from low sea levels
could have been wiped out as the sea rose again. These changes leave
gaps in the fossil record, making some species appear to be extinct
before they actually are, or erasing the record of existence of
some species entirely. Moreover, the incompleteness of the fossil
record makes it difficult to distinguish between species and varieties
by using fossils alone, as many intermediate species have been lost
due to geological changes. Darwin argues that the fossil record
is intermittent, and scientists cannot rely on it to explain either
the totality of species existing at a time or the exact orders and
dates of extinction.
On the other hand, the intermittent nature of the fossil
record helps explain one of the potentially fatal flaws of Darwin’s
theory: If slow modification of species occurs through natural selection,
why do paleontologists see the abrupt appearance of whole groups
in the geological record without any sign of intermediate forms?
The imperfection of the geological record, Darwin suggests, means
that scientists can neither make an assumption about the date of
species formation according to the position of species’ remains
in the fossil record, nor assume that intermediate forms never existed
just because the species’ remains could not be found. Even if species
of the same group were found together in strata of fossil-bearing
rocks, scientists would not be able to conclude that these species
formed together, because changes in the geological composition of
the earth may have shifted preexisting formations. Scientists know
too little about the history of the earth to refute Darwin’s theory
simply based on the imperfect geological record.
Analysis
Darwin once again draws on existing uncertainties in scientific knowledge
to defend his theory of natural selection. Some naturalists dispute
Darwin’s theory because of its inconsistencies with the geological
record. However, Darwin turns these criticisms around, arguing that
the real uncertainty is not in his theory of natural selection but
in the geological record itself. He demonstrates just how much remains
unknown about the history and geology of the earth by pointing out
the mysteries of geological change—for example, the speed at which
sedimentation occurs, the ability of fossil formations to withstand
decay, and the broad changes in land formation that affect the preservation
of fossils. Darwin argues that uncertainties in geological and paleontological
theories should not result in the disqualification of his own scientific
inquiry and theorizing.
However, Darwin’s argument about the uncertainty of the
geological record is only partially correct. Future discoveries
in science would elucidate some of the uncertainties plaguing his
work. Geologists learned more about changes in the earth’s surface,
patching up inconsistencies in the geological record. Mendelian
genetics and mutation theory explained that the appearance of variations,
leading to species formation, was a sudden process—not the gradual process
of intermediate forms, as Darwin argued in his theory. On the other
hand, mutation theory also confirmed that natural selection did,
in fact, occur from the appearance of variations in a species—although
that appearance happened suddenly, rather than gradually, as Darwin
had theorized. If a variation occurred that was advantageous to
the organism, the organism would have a better chance of survival
and would be more likely to reproduce, spreading the advantageous
variations to its offspring and creating a new species. Aside from
the speed of the process, this is precisely what Darwin had argued.
