Summary
Here, Darwin addresses the problem that the sterility
of hybrid species poses to his theory of natural selection. When
a breeder or scientist crosses two species, the offspring, known
as hybrids, are often sterile. Darwin says hybrids are sterile because
their reproductive systems are incorrectly formed. “Pure” species
have perfectly designed reproductive systems, adapted to their particular
species. Different species have developed different reproductive
systems, which is why it is often difficult to get two separate
species to conceive together in the first place. When a hybrid offspring
is produced, the hybrid’s reproductive system will be imperfect,
because it is a cross between two different reproductive systems
rather than one perfectly adapted one. Even if the first hybrid
generation remains fertile, the second generation of hybrids will
often be sterile. Crossing species, in other words, alters the perfectly
developed reproductive systems in individual species, making sterility
a likely outcome.
Darwin uses the discussion of the sterility of hybrids
to refute a common claim made by scientists of the time: that the
fertility of the offspring of two parents determines whether those
parents are of the same or different species. Some scientists believe
that parents who have fertile offspring must be of the same species,
and that parents who have infertile offspring must be of two different
species. Conventional wisdom had it that the division between species
and varieties could be determined by examining the fertility of
offspring. Darwin calls this conventional wisdom into question.
The fertility experiments scientists were doing on hybrids, for
example, may have been tainted by close interbreeding between offspring
of the same parents. In these experiments, familial interbreeding,
not the hybridization of the parents, may have been the cause of
offspring infertility. Darwin also notes cases in which the pollen
of a different species of plant fertilizes certain plants to a greater
extent than the pollen of a member of the same species. Some varieties
of plants, assumed to be members of the same species, have proven
to be infertile when crossed. Darwin argues that oftentimes the
reasons for the sterility of a particular organism are unknown.
He further argues that if scientists place too much emphasis on
using sterility to determine species, without knowing the precise
causes of sterility, they risk coming away with an incorrect definition
of species and varieties.
The main problem with drawing conclusions based on sterility
is that so little is known about the reproductive systems of many
species. One scientific misconception, which Darwin calls “systematic affinity,”
is that different species can successfully produce fertile offspring
simply because of their structural and physical similarities. However,
Darwin notes that there are numerous examples of species that are
unable to cross despite their systematic affinity, such as the stallion
horse and the female ass. The ability of two species to cross successfully
is related to the similarity between their reproductive systems,
not to their external appearances—and Darwin claimed that scientists
did not know enough about the reproductive systems of most species
to predict with certainty which species they could successfully
cross. The ability of species and hybrids to reproduce successfully
is dependent on unknown variables. Darwin maintains that science
has no reliable way to distinguish between species and varieties.
Analysis
At first glance, this chapter seems out of place. Why,
we might wonder, would Darwin address the interbreeding of species
and the creation of hybrid species? After all, he does not claim
that crossing two existing species leads to the creation of new
species—this would introduce a new mechanism of evolution and jeopardize
his natural selection theory altogether. His intention in discussing
hybrids and cross-breeding is to address critiques of his descent
with modification theory. He argues that organisms’ variations perpetuate
in subsequent generations, causing divergence from their parent
species and eventually creating entirely new species. At the time,
scientists believed that species were unable to produce fertile
offspring when crossed. They wondered, then, how a simple variation
could eventually create this barrier in fertility, leading to the
creation of new species. To prove descent with modification, Darwin
had to explain how varieties cross the line into species.
Unfortunately, Darwin does not have a good explanation
for how variations might lead to the fertility divide between species. Rather
than attempt to explain sterility, Darwin chooses to refute the
accepted definition of species entirely, arguing that sterility
is simply not a good way to distinguish between varieties and species. Darwin
has already made the similar argument that no clear distinction
between varieties and species exists. Maintaining this argument allows
Darwin to avoid identifying a mechanism for how varieties become
species—they simply do, without having to pass a specific threshold
of difference from the original species. Curiously, Darwin does
not hypothesize that natural selection produces slight modifications
in the reproductive systems of certain varieties (eventually rendering
them unable to reproduce with other groups, and thus creating new
species under the fertility definition of species). Unfortunately,
Darwin’s refutation is scientifically incorrect. Even today, naturalists
define a species as a group of organisms that can produce fertile
offspring. Essentially, Darwin leaves the question of how varieties
become species unanswered.
Despite its flaws, Darwin’s analysis remains important
as a model for scientific critique. Just as he uses specific examples
to draw general conclusions about scientific principles and to argue
his own theory of natural selection, he illustrates how specific
counter-examples can be used to refute other scientific theories.
Darwin uses examples of fertile hybrids and infertile non-hybrids
to question the consistency of fertility and infertility in a species,
which scientists used to provide a general definition of species.
Moreover, in locating the source of infertility in the reproductive
system rather than in the external appearance of species, Darwin
illustrates the specificity necessary in forming scientific theories.
In an additional critique of the scientific definition of species,
Darwin proves that the internal nature of the reproductive system
makes its physiology uniquely unknown to science. Despite the continued
acceptance of the fertility definition of species, Darwin demonstrates
the limits of scientific inquiry by critiquing these gaps in knowledge.
