The Origin of Species · Chapter
On the Intercrossing of Individuals explained
Darwin argues that an occasional cross between distinct individuals is a general law of nature for all organic beings, including hermaphrodites.
What happens
Darwin argues that an occasional cross between distinct individuals is a general law of nature for all organic beings, including hermaphrodites. He presents evidence from plants and animals, showing that cross-fertilization increases vigor and fertility, while self-fertilization diminishes them. He describes adaptations in flowers that promote cross-pollination, such as dichogamy and insect-mediated pollination, and notes that even in hermaphrodites, mechanisms exist to facilitate occasional intercrossing.
Themes in this chapter
Variation and Inheritance
Darwin uses the vigor and fertility of crossbred offspring versus the diminished vigor from close interbreeding to argue for the necessity of occasional crosses.
Adaptation and the Utilitarian Doctrine
Floral adaptations such as dichogamy, insect-attracting structures, and mechanisms preventing self-fertilization are presented as evidence for the utility of cross-pollination.
Characters to notice
- Charles Darwin
Author and narrator; presents his own experiments and observations on cross-fertilization.
- Sprengel
Cited for early suggestions on cross-fertilization and for observations on floral contrivances.
- Kölreuter
Mentioned for his work on the barberry and on cross-fertilization.
- Gärtner
Referenced for demonstrating that a plant's own pollen is prepotent over foreign pollen when crossing distinct species.
- Professor Huxley
Consulted as an authority on hermaphrodite animals and the impossibility of self-fertilization in enclosed organs.
Key passages
“I have collected so large a body of facts, and made so many experiments, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature that no organic being fertilises itself for a perpetuity of generations; but that a cross with another individual is occasionally—perhaps at long intervals of time—indispensable.”
Darwin argues that his extensive experiments and observations, along with common breeder knowledge, show that crossbreeding increases vigor and fertility while inbreeding reduces them, leading him to conclude that no organism can self-fertilize indefinitely and that occasional cross-fertilization is necessary.
Central thesis of the chapter: the necessity of occasional intercrossing.
“Insects act like a camelhair pencil, and it is sufficient, to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if a plant’s own pollen and that from another species are placed on the same stigma, the former is so prepotent that it invariably and completely destroys, as has been shown by Gartner, the influence of the foreign pollen.”
Darwin explains that insects transfer pollen between flowers like a brush, but this rarely creates hybrids between species because a plant's own pollen outcompetes foreign pollen, as Gartner demonstrated.
Clarifies the role of insects in pollination and the prepotency of own pollen over foreign pollen.
“From these several considerations and from the many special facts which I have collected, but which I am unable here to give, it appears that with animals and plants an occasional intercross between distinct individuals is a very general, if not universal, law of nature.”
Darwin concludes that based on his collected evidence, occasional cross-fertilization between different individuals is a widespread, possibly universal, natural law.
Final conclusion of the chapter.
