On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 10 : On the Imperfection of the Geological Record | Summary

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Summary

In this chapter Darwin discusses the geological record in detail, focusing on why there are so few "intermediate links" between different species. He draws on three points, referring heavily to Lyell and his work, Principles of Geology. First, the geological record was formed over a long period of time. Second, the record is imperfect, due to the shifting of the crust and variations in how geological strata are formed. Third, not all animals are able to become fossils—for instance, soft-bodied animals will only become fossilized in very specialized conditions.

Darwin notes while he is unsure of the time frame required for a new species to form, he suggests it is longer than the time depth represented in many geological strata. He notes the difficulty for naturalists in distinguishing species from varieties in existing organisms. In contrast, many paleontologists have distinguished species based on small characteristics on a limited number of fossil specimens.

Darwin addresses one of the potential critics levied at his theory of natural selection based on the fossil record. He notes paleontologists have identified instances where "whole groups of species" appear. However, he attributes this "abrupt" appearance to the incompleteness of the geological record. He states the greatest "difficulty" is in accounting for the age of the earth and notes the degree of variation in species during the Cambrian period (541–485.5 million years ago) suggests an age for the earth exceeding the posited age of 20 to 400 million years. Darwin admits he can provide "no satisfactory answer" to this problem. In conclusion he notes while many geologists and paleontologists originally "maintained the immutability of species," they have since revisited this idea. Darwin states the theory of natural selection is a logical extension of the processes described in Lyell's work.

Analysis

The fossil record is inherently biased toward species with hard parts and is inherently spotty. In many cases direct observation of species cannot be made as the fossil may be incomplete or may not represent the varied structure. These facts have led to arguments regarding the lineages of different species and often result in multiple hypotheses. For instance, the discovery of Homo naledi in 2013 led to numerous hypotheses as to where the hominid fit into the line of human evolution. Darwin argues these facts about the fossil record explain why there are so few transitional species represented.

In this chapter Darwin also discusses the age of the earth—a topic that would not be fully addressed until the 20th century. Darwin correctly hypothesizes that, based on the theory of natural selection, earlier "ancestral" species existed prior to the Cambrian period. However, in On the Origin of Species, he takes issue with this supposition, as it would greatly extend the age of the earth. While at the time of Darwin's work, the age of the earth was believed to be around 400 million years old, scientists today agree it is approximately 4.7 billion years old. Although he questions his own conclusions, Darwin is correct in assuming ancestral species existed prior to the Cambrian period. Scientists now know there were species in the Precambrian period that were ancestors to those living in the Cambrian period.

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