Chapter 4 of Darwin’s Origin of Species
is a crucial chapter, outlining his views on Natural Selection, but it is also 51 pages long. In this video, I will give a breakdown of
the 8 different sections of this chapter, and say a few words about what you should
focus on in reading each section. The chapter begins with 8 pages comparing
natural selection to artificial selection. This is pages 80-87 in the First Edition.
In the first paragraph, Darwin defines Natural Selection as “the preservation of favourable
variations and the rejection of injurious variations.”
Key points of this section include: that natural selection is capable of even more powerful
and more profound transformations than artificial selection by selective breeding, that natural
selection is capable of producing an immense diversity of organisms, and the survival of
only the best-adapted ones, that natural selection adaptations are gradual, almost too gradual
to be observed, and that natural selection can only select for traits that increase the
survivability of the individual possessing them—altruistic or self-sacrificial traits
are going to be a problem for this theory. The second section is 3 pages of comments
on what Darwin calls “Sexual Selection.” The editor, James Costa, notes that here Darwin
recognizes that “reproduction is the ultimate evolutionary currency.”
Main points include: that within each species, males compete against other males to mate
& reproduce, and females compete against other females. Both these selection processes work
to improve and transform the species from generation to generation.
The third section is 5½ pages of illustrations of Natural Selection in action. Darwin describes
at length some observed cases that show his theory and how it works. These cases include
hunting wolves and the pollination of flowers by bees.
The fourth section is 6 pages long, dealing with intercrossing, or out-breeding.
It was known in Darwin’s time that repeated inbreeding caused weak and sickly offspring,
and that occasionally crossing varieties with more-distant varieties led to increased health
and vigor. Darwin gives several examples illustrating and supporting this conclusion.
The fifth section is 7½ pages on circumstances favorable to Natural Selection, which Darwin
calls “an extremely intricate subject.” He discusses the effects of various factors
on natural selection. These include: a large number of heritable variations, intercrossing,
physical isolation, and largeness of area in which the species dwells.
The sixth section is a 2 page treatment of extinction. Darwin considered the factors
and mechanisms of species extinction. This short section is worth a careful read-through.
The seventh and longest section is 16 pages, and includes the only illustration in the
book, a chart. It deals with “divergence of character,” the way competition between
similar species eventually produces separate distinct species.
Major points include: the mechanism of divergence, with examples from nature and from artificial
selection, and the benefits of divergence and species diversification, again with many
examples. Darwin then explores the way divergence of
character combines with natural selection and extinction, and this is where he uses
his chart. The remainder of this section, pages 116-126, along with the included chart,
to which it frequently refers, is worth your careful attention.
The final section is 3½ pages long, and it is a summary of the entire chapter. The summary
provides a good overview of Darwin’s argument so far, and I urge you to read it carefully,
both before and after you read the entire chapter.
I hope you have found this breakdown of the sections of Chapter 4 helpful. Thanks for
watching today; goodbye.