The Origin of Species · Chapter
Reciprocal Dimorphism and Trimorphism explained
Darwin discusses the phenomenon of dimorphic and trimorphic plants, which have two or three distinct forms differing in the lengths of pistils and stamens.
What happens
Darwin discusses the phenomenon of dimorphic and trimorphic plants, which have two or three distinct forms differing in the lengths of pistils and stamens. He shows that full fertility requires legitimate unions between corresponding forms, while illegitimate unions result in varying degrees of sterility, closely resembling the behavior of distinct species when crossed. The chapter argues that sterility in such cases is not a reliable test of specific distinction and suggests that it depends on the nature of the sexual elements rather than structural or constitutional differences.
Themes in this chapter
Variation and Inheritance
The chapter explores how different forms within the same species exhibit variation in reproductive organs and fertility, and how these traits are inherited in illegitimate offspring.
Adaptation and the Utilitarian Doctrine
The legitimate unions are shown to be the most fertile, suggesting an adaptive advantage to the structural correspondence between stamens and pistils.
Characters to notice
- Charles Darwin
Author and primary investigator; conducted experiments on dimorphic and trimorphic plants over four years.
- Gärtner
Cited as an excellent observer who concluded that species sterility is due to differences in the reproductive system.
Key passages
“Now I have shown, and the result has been confirmed by other observers, that in order to obtain full fertility with these plants, it is necessary that the stigma of the one form should be fertilised by pollen taken from the stamens of corresponding height in another form.”
Darwin demonstrated that for these plants to be fully fertile, pollen must come from stamens that match the height of the pistil's stigma.
Key finding on legitimate fertilization in dimorphic and trimorphic plants.
“It is hardly an exaggeration to maintain that illegitimate plants are hybrids, produced within the limits of the same species by the improper union of certain forms, while ordinary hybrids are produced from an improper union between so-called distinct species.”
Illegitimate plants are essentially hybrids formed within a single species, just as ordinary hybrids form between different species.
Darwin draws a parallel between intraspecific illegitimate unions and interspecific hybridization.