In Chapter 5 of the Origin, Darwin turns to what he admits is the least understood and most mysterious part of his theory: the origin of variation. He opens with a candid confession. When he has previously referred to variations arising by chance, this was not to suggest randomness in the sense of uncaused events, but rather to acknowledge ignorance. The causes of variation were largely unknown in Darwin’s time, and this chapter makes no attempt to conceal that fact. Instead, it explores the patterns and recurring associations that Darwin observed, offering a collection of empirical insights rather than a unified theory. These reflections are grouped under what he called the “laws of variation,” though they are perhaps better understood as tentative rules and informed speculations.
Darwin was writing well before the birth of modern genetics, and without a clear understanding of how traits are inherited, his grasp of variation was inevitably shaped by the dominant views of the time. He mixes together two fundamentally different mechanisms: Those that generate new genotypes through recombination and mutation, and those that shape phenotypes through development and environmental influence. He recognized that selection acts on phenotypes, not on the invisible heritable units themselves, but the boundary between these layers was still unclear.
The idea of inheritance itself was still under debate. Lamarckian inheritance, the idea that traits acquired during an individual’s lifetime could be passed on to offspring, was widely accepted, and Darwin did not fully reject it. He entertained the possibility that use and disuse could lead to evolutionary change. This view would later be challenged by molecular biology, though recent advances in epigenetics have shown that environmentally induced changes in gene expression can in some cases be inherited. Darwin didn’t have the tools to explore this, but he was open to the possibility that the environment could influence heredity.
Several striking examples illustrate Darwin’s point. He believed that cave animals, many of which are blind, had lost their vision through disuse. With no need to see in perpetual darkness, the pressure to maintain eyesight was relaxed, and natural selection no longer preserved the trait. Similarly, Darwin discusses the flightlessness of many island insects. On Madeira, he reports that out of 29 beetle genera, 23 include species that have wings so reduced they cannot fly. Darwin suggests that flightlessness in this context may have evolved because flying insects are more likely to be blown out to sea. Selection thus acts indirectly, not by favoring flightlessness per se, but by eliminating individuals who fly and perish.
Darwin also considers the phenomenon of acclimatization, where species are capable of surviving in climates different from those in which they evolved. He observes that some plants and animals can adapt relatively easily to new environments, while others are bound tightly to their native conditions. He even speculates about what determines a species' range limits and the existence of species borders, especially in the absence of obvious geographic barriers. This insight anticipates modern ecological research on environmental tolerances and range shifts under climate change.
Importantly, Darwin emphasizes that a species’ geographic range is not shaped solely by climate. He writes that species are often limited quite as much as, or more than, by the competition of other organic beings. Adaptation alone does not guarantee success; the presence of other species competing for the same resources can be just as decisive. He also notes that species ranges can shift over time without clear explanations. The spread of species like the serin finch or the collared dove in Europe, or the house finch in North America, are examples of such range expansions that remain difficult to fully explain, even today.
Darwin then turns to a theme he revisits repeatedly: correlation of growth. He argues that the parts of an organism are interconnected in such a way that when one part changes (through natural selection or other causes), other parts often change as well. This idea of the organism as an integrated whole underpins Darwin’s evolutionary thinking. For example, in the evolution of the horse, the shift from browsing to grazing on tough grasses led to selection for longer, more durable teeth. But this change also required modifications in the jaw, skull, and digestive system. Selection may act locally, but it has systemic consequences.
Some of these correlated changes, Darwin notes, arise from what would now be called pleiotropy, where a single gene influences multiple traits. He documents various cases of unexpected associations: white cats with blue eyes are often deaf; the calico coat pattern in cats is typically female, which we now know is due to X-chromosome inactivation. Darwin may not have known the underlying mechanisms, but his empirical eye captured recurring links that would later become key to understanding genetic architecture.
Another major theme is the idea of reversion. Darwin observes that when highly specialized breeds are crossed, traits from a common ancestor sometimes reappear. For example, crossing different pigeon varieties can result in offspring with traits reminiscent of the ancestral rock dove. He uses this to argue that underlying genetic material may persist across generations, reemerging when conditions align. In one particularly famous case, he recounts how a chestnut mare bred with a male quagga produced offspring with leg stripes more distinct than those of a pure quagga. Such atavisms, he suggests, offer glimpses into evolutionary history.
Toward the end of the chapter, Darwin addresses some of the more speculative and controversial ideas of his time. One of these is telegony: The belief that a male’s reproductive influence could persist in a female’s subsequent offspring, even if he was not the father. To explain this and other anomalies, Darwin proposed a provisional hypothesis of heredity: pangenesis. According to this idea, all parts of the body shed tiny particles, or gemmules, which gather in the reproductive organs and influence offspring traits. These gemmules, he suggested, could carry information about acquired characteristics and environmental conditions, offering a way to unite Lamarckian inheritance with variation and selection.
Though pangenesis was never accepted, and later disproven, it had lasting influence. Darwin’s term gave rise to Hugo de Vries’s pangene, which was later shortened by Wilhelm Johannsen to the now-familiar term gene. Johannsen also coined the words genotype and phenotype, finally separating hereditary information from its expression. Darwin never made this conceptual distinction, but his observations laid the groundwork for those who would.
In the final pages, Darwin returns to a theme of scientific humility. He concedes that our ignorance of variation is profound. Not in one case out of a hundred, he writes, can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents. But he argues that this lack of understanding should not dissuade us. The mechanism of natural selection, he insists, does not depend on knowing how variation arises. So long as heritable variation exists and the natural world is full of it, selection will shape populations over time.
Darwin ends the chapter with one of the most powerful claims in the book. However variation may originate, it is through the steady accumulation of such differences that nature has crafted all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive. The causes of variation may be obscure, but the consequences are everywhere.