The Origin of Species · Chapter
Laws Governing the Sterility of First Crosses and of Hybrids explained
Darwin examines the complex laws of sterility in first crosses and hybrids, arguing that sterility is not a specially endowed quality to prevent species from blending but rather an incidental consequence of unknown differences in reproducti
What happens
Darwin examines the complex laws of sterility in first crosses and hybrids, arguing that sterility is not a specially endowed quality to prevent species from blending but rather an incidental consequence of unknown differences in reproductive systems. He draws parallels between grafting and crossing, showing that both follow systematic affinity but are not strictly governed by it.
Themes in this chapter
Variation and Inheritance
The variability of sterility in first crosses and hybrids, and its independence from external resemblance, underscores the role of unknown reproductive differences.
Adaptation and the Utilitarian Doctrine
Darwin challenges the idea that sterility was specially designed to prevent interbreeding, arguing instead that it is incidental—a non-adaptive byproduct.
Characters to notice
- Gärtner
Darwin draws heavily on Gärtner's work on plant hybridisation to illustrate the gradation of fertility and the rules governing crosses.
- Charles Darwin
Darwin presents his argument that sterility is incidental, not specially endowed, using examples from grafting and crossing.
Key passages
“that it is by no means always the same in degree in the first cross and in the hybrids produced from this cross; that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent; and lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other.”
The degree of sterility in first crosses and hybrids varies independently, is unrelated to parental resemblance, and does not strictly follow systematic affinity.
Darwin summarizes key anomalies in the laws of crossing that challenge the idea of special endowment.
“To grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems a strange arrangement.”
It would be odd if species were given the ability to hybridize but then prevented from producing offspring by sterility that doesn't match the ease of the initial cross.
Darwin uses rhetorical questioning to argue against special creation of sterility.
“As in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees belonging to quite distinct families; and, on the other hand, closely allied species and varieties of the same species, can usually, but not invariably, be grafted with ease.”
Just like in hybridization, grafting success is generally limited by how closely related the plants are, though there are exceptions.
Darwin draws a parallel between grafting and crossing to support his incidental theory.