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.
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.