In Chapter 8 of the Origin, Darwin turns to the question of hybridism: One of the most difficult and problematic issues for his theory. Unlike earlier chapters, where the gradual accumulation of evidence builds toward a coherent argument, this chapter presents a landscape of exceptions, contradictory data, and theoretical uncertainty. Darwin was working without a science of genetics, without knowledge of chromosomes, and without a clear understanding of how heredity operated at a cellular level. Despite these limitations, his analysis remains thoughtful, precise, and importantly experimental.
Darwin begins with the common assumption of his time: that hybrids between distinct species are typically sterile, while hybrids between varieties of the same species are usually fertile. If this were universally true, it would offer a neat criterion for distinguishing between species and varieties. However, Darwin quickly dismantles this notion through extensive evidence, much of it gathered from plant studies. He points out that many distinct plant species cross easily and produce fertile offspring, while some varieties of the same species can be unexpectedly sterile when crossed. This variability undermines any attempt to draw a clear line between species and varieties based on fertility alone.
Darwin draws heavily from the experimental work of two German botanists, Joseph Kölreuter and Karl Friedrich von Gärtner, who had conducted hundreds of hybridization experiments. Yet despite their careful methods, the two scientists often reached opposite conclusions about the same species. In ten specific cases, Kölreuter found that supposedly distinct species produced fertile offspring, prompting him to reclassify them as varieties. Gärtner, using what Darwin argues were flawed methods—such as castrating flowers and growing plants indoors, found the same crosses to be sterile and insisted they were distinct species. For Darwin, this disagreement between the two most experienced hybridists of the time is proof that sterility cannot serve as a reliable marker for species boundaries.
The heart of the chapter lies in Darwin’s rejection of hybrid sterility as an inherent or divinely imposed barrier between species. Instead, he proposes that sterility is an incidental by-product of divergence, particularly in the reproductive systems of organisms. When animals or plants are removed from their natural conditions, or when hybrids are formed between forms that have long evolved separately, their reproductive systems may malfunction. This disruption, rather than any intrinsic rule of nature, explains the sterility of many hybrids.
Darwin extends this logic to domesticated animals. He notes that if many of our domestic breeds descended from multiple wild species, as he believed, then hybrid sterility must have been reduced over time through continuous interbreeding. The very existence of fertile domestic breeds, in other words, is evidence that sterility is not fixed or fundamental. To bring more structure to the discussion, Darwin devotes over twenty pages to analyzing what he calls the “laws” of hybridism. These include:
The sterility of first crosses and the hybrids resulting from them
The likely causes of such sterility
The fertility of varieties and their mongrel offspring
The morphological similarities between hybrid generations, regardless of fertility
These sections are dense and technical, but they serve an important purpose. Darwin wants to test whether the patterns of sterility observed in nature reflect an intentional design to prevent species from blending, or whether they are simply a consequence of evolutionary divergence. Unsurprisingly, he concludes the latter. The data do not support the idea that sterility is a special endowment given to species to keep them separate. Instead, it is the result of accumulated modifications, especially in the reproductive system, after long periods of independent evolution.
He repeatedly emphasizes that while some degree of sterility in hybrid crosses is common, it is by no means universal. In fact, the variability of these outcomes is itself an argument for gradual evolution. If all hybrids were always sterile, or always fertile, it might suggest fixed categories. But the spectrum of outcomes points to continuous change. Darwin sums this up by saying that sterility is not a special endowment, but is incidental on slowly acquired modifications.
One of the more subtle arguments in the chapter comes when Darwin compares hybrids (crosses between species) and mongrels (crosses between varieties). Breeders in Darwin’s time agreed that second-generation hybrids were often more variable than first-generation ones, and Darwin notes that this is also true of mongrels. He highlights the general similarity between the offspring of crossed species and crossed varieties: both tend to produce intermediate types. This similarity is crucial. If species were specially created and varieties evolved by secondary laws, then the parallel outcomes would be baffling. But if both species and varieties arise through the same gradual processes, then the resemblance is exactly what we should expect.
The final pages of the chapter return to Darwin’s central theme: there is no essential distinction between species and varieties. The degree of fertility in hybrid crosses roughly correlates with relatedness, but this is not an absolute rule. Sterility increases with genetic divergence, but it is a consequence, not a cause, of speciation. It is not something imposed to preserve fixed species boundaries: It is simply a by-product of evolutionary history.
What stands out in this chapter is Darwin’s method. Faced with complex, often contradictory data, he doesn’t dismiss or obscure the problems. He gathers observations, raises precise questions, and proposes testable explanations. When he encounters gaps in the existing literature, he designs his own experiments to fill them. The chapter’s value lies not just in its conclusions, but in how Darwin works through uncertainty, how he thinks scientifically in a time before genetics, before chromosomes, before molecular biology.
Even if many of his specific claims are now outdated, the structure of his reasoning holds. Darwin does not need perfect knowledge to argue persuasively. He needs only to show that hybridism, like all other biological phenomena, is best understood through descent with modification and the slow, imperfect action of natural selection.