Note: This literature guide is based on the
Barnes & Noble Classics edition of Origin of Species. This
edition, like others, omits Chapter VII: Miscellaneous Objections
to the Theory of Natural Selection, which is included in some versions
of the book.
Summary
Darwin discusses the heritability of instinct and its
role in natural selection. Instinct proves difficult
for Darwin to define. It is similar to habit, because it consists
of actions performed repeatedly by an individual animal. But unlike
habit, which Darwin believes animals learn, instinct is inherited.
The causes of innate instincts remain unknown, in the same way the
causes of physical variations are unknown. Darwin believes that
inherited habits—those learned by a parent and subsequently passed
on to offspring by hereditary inheritance—may play a role in the
construction of these instincts. However instincts come to be, Darwin
argues that natural selection acts on them just as it acts on physical
variations. If an instinct is advantageous to a species’ survival,
natural selection allows organisms with that instinct to survive
over others and perpetuate that instinct in their offspring. Thus,
natural selection helps create entire species with well-adapted
instincts, allowing them to survive in a variety of natural environments.
Darwin provides many examples of species that have innate, advantageous
instincts. Certain hens, for example, have exhibited an instinct
for laying their eggs in another hen’s nest, allowing them to give
birth to numerous offspring without having to take care of all of
them. In certain ant populations, some ants are born with “slave”
instincts. These ants instinctively care for the rest of the population
without any type of training after their birth. Hive bees instinctively
construct the honeycomb to hold the greatest amount of honey possible.
Darwin provides the results of a number of experiments in which
the hive bees shaped whatever wax Darwin gave them, no matter what
size or shape, in the same way. The results demonstrate that the
construction of the hive holes is instinctual for the species. All
of these instincts, Darwin argues, come about as a result of slight
modifications over time: Natural selection perpetuates the most
advantageous instincts, gradually honing them into their most advanced
state in the latest incarnation of a species.
In concluding this chapter, Darwin presents one complication
to his theory of the natural selection of instincts, a complication
that, in his mind, almost destroys the entire theory. In some ant
species, the worker ants, which vary from the nonworker ants in
both structure and instinct, are sterile. If these ants are unable
to produce offspring, how could their traits be inherited by subsequent generations?
Moreover, a number of different structures for these infertile ants
exist. How could all of these different structures be passed down
through natural selection if the carriers of these structures were
always infertile? While Darwin admits how difficult these questions
are to answer, he argues that natural selection works on entire
species, not just on individual organisms. Therefore, natural selection
must have perpetuated the fertile ants in this species, the ones
who carried a tendency to produce these sterile members, which proved
advantageous to ant society by providing it with worker ants. The
parent ants that were able to produce infertile offspring proliferated—not
the sterile worker ants themselves. This difficult case illustrates
the power of natural selection in selecting characteristics that
could have been acquired only through variation and not through
habit.
Analysis
In his continued explanation of natural selection, Darwin
discusses the heritability of instinct, shifting focus from the
selection of physical characteristics to the selection of mental
characteristics. This analysis builds on his previous discussion
of natural selection but adds a new dimension: the power of mental
characteristics (or instincts) to shape the survival and proliferation
of species. Clearly, mere physical strength or structural adaptation
is not enough for animals to survive in nature. Animals must know
what to do with their physical adaptations and how to use them to
live in their environment. They must know how to gather food, build
shelter, and hide from their enemies. While the necessity of instincts
may seem obvious, in this analysis, Darwin reminds the reader that
advantages gained from instincts also become part of natural selection’s shaping
of species.
Darwin’s discussion of advantageous mental characteristics
in animals implies that organisms have an innate intelligence, and
that some use this intelligence to survive better in nature. Darwin
argues that the smartest animals are most likely to win out in natural
selection, because their intelligence is advantageous to their survival. Indeed,
many of the instincts he cites as examples, such as the mathematical
precision of hive bees as they make honeycombs, are quite sophisticated.
Darwin’s theory of heritability and natural selection of instincts
implies that a form of intelligence plays a role in the development
of the human species. Social Darwinists would later apply Darwin’s
theory of natural selection to questions of human intelligence and
how it shapes the human population. They would ask, Is natural intelligence
in human beings instinctive and therefore innate, because it is
passed on through heredity by innately intelligent parents? Do more
intelligent human beings stand a greater chance of survival? Are
intelligent humans naturally selected to survive and flourish in
the human population?
Although Darwin admits that habit may play some unknown role
in the development of variation, he provides an example of a hereditary
characteristic—the birthing of sterile worker ants—that would be
impossible to shape by habit or choice. Since the characteristic
of sterility in worker ants is clearly inherited, some other means
of creating variations must exist for these variations to be passed
down to subsequent generations. Although Darwin cannot pinpoint
the means by which these variations are created—Mendelian genetics
and genetic mutation theory would provide those answers in the twentieth
century—his example of the sterile worker ants provides a direct
refutation of Lamarck’s evolutionary theory. Darwin points this
out by asking how an instinct to reproduce sterile ants could be
produced through chosen habit. Darwin’s “difficult case” of the
sterile worker ants, which he believes could have undermined his
own theory, ends up strengthening it instead by refuting the central
assumption of Lamarckian evolution—that habits are the cause of
evolution.
