Darwin notes that domestic species contain much more variety in their different subsets and races than species found in nature. Scientists have traditionally attributed variations in species to environmental factors, such as the availability of food or the climate and heat conditions in which species types are raised. However, Darwin rejects the idea that environmental factors are the primary triggers of variation, because new varieties of even the oldest known species have come into being without a change in environmental conditions. Darwin also rejects the theory that habits of use (of bodily structures, for instance) are the main cause of variation. While he notes that in some species types the use of particular organs has yielded different body structures (such as the strong legs of ducks that don’t fly versus the strong wings of those that do), far starker variations exist that defy environmental explanation.

The most important explanation for variation is reproduction. Parents pass specific variations on to their offspring, and those variations are perpetuated in subsequent generations. This explains why rare traits occasionally pop up in individual plants or animals and then appear again in those organisms’ direct offspring. Darwin admits that scientists don’t yet fully understand the laws of inheritance as they apply to parents and their offspring. For example, scientific theory has not explained why offspring inherit traits on some occasions but not others, or why certain traits skip generations entirely, appearing in parents but not offspring, and then reappearing in the offspring’s offspring.

Furthermore, it remains unclear in many cases whether species types are descended from one parent species (the original species that gave birth to the others) or from several different parent species. In his study of various breeds of domestic pigeons, Darwin found that all pigeons share similar coloring and body structure. Additionally, after cross-breeding several types of pigeons, he discovered that the resultant hybrids were “perfectly fertile,” which indicates that they descended from one parent species, as the offspring of two distinct breeds are sterile. These findings suggested that, despite wide variation in the types of pigeons, all must have descended from the rock pigeon. Darwin concludes that most species types may, in fact, have descended from one parent, rather than multiple parent species, as breeders had previously believed.

Darwin discusses the breeding of domestic species and man’s influence on the creation of new races and subspecies. While nature produces variations, in controlled environments man has the power to select which variations will survive and influence future generations. In a process known as conscious selection, a domestic breeder selects the best or most useful types within a species and breeds them over other types. This breeding creates a large stock of those particularly useful types that are then more readily available to breed in subsequent generations.

Domesticated species are also subject to unconscious selection, a process by which characteristics become predominant without the aid or influence of breeders. Gradual changes, often unnoticeable at first, can account for differences in breeds over time, such as the increased speed and size of English racehorses compared to their original Arab stock. Valuable traits may be arbitrary (related to beauty) or unarbitrary (such as size, weight, speed, or other traits that may prove advantageous to the species). These unconsciously selected traits may benefit the species types in unknown ways, perhaps by allowing them to withstand famine or accidents in order to reproduce and perpetuate the traits in their offspring. The gradual nature of this process may explain why the original parent stock of a species is difficult to determine: Since changes occur slowly over time, two stocks that were once related can look quite different after many generations.


Darwin’s major claim in Chapter I is that heredity is the key to explaining variation. This argument contrasts slightly with the arguments made by other evolutionary theorists, such as Jean-Baptiste Lamarck, who claimed that environmental factors are the best explanation for the development of species. Darwin does not dismiss these earlier theories. In fact, he acknowledges that environmental factors do play a role in species type differentiation. His own example—the strong wings of flying ducks versus the strong legs of non-flying ducks—supports the idea that environmental factors affect species type development. Nonetheless, in arguing that heredity plays the larger role, Darwin implicitly criticizes Lamarck’s theory even as he recognizes its importance. He asks, If traits are passed through heredity rather than developed through environmental use, can developed traits such as strong wings then be passed on through heredity? Or must wing strength already be a hereditary trait in itself? If strong wings are hereditary, how is it that those ducks that fly are lucky enough to have this trait in their hereditary makeup? Darwin poses these questions to acknowledge that some of the mechanics of heredity remain unknown.