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.
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.