The Origin of Species · Chapter
IX: Hybridism explained
Darwin examines the sterility of first crosses between species and of their hybrid offspring, arguing that this sterility is not a special endowment acquired through natural selection but an incidental result of differences in the reproduct
What happens
Darwin examines the sterility of first crosses between species and of their hybrid offspring, arguing that this sterility is not a special endowment acquired through natural selection but an incidental result of differences in the reproductive systems of the parent species. He contrasts the fertility of varieties (which are generally fertile when crossed) with the sterility of species, and discusses the complex gradations of sterility, the role of early embryo death, and the parallel between grafting and crossing difficulties. He concludes that the facts do not oppose the view that species originally existed as varieties.
Themes in this chapter
Natural Selection and the Struggle for Existence
Darwin argues that sterility of first crosses and hybrids cannot have been acquired through natural selection, as it offers no advantage to individuals.
Variation and Inheritance
The chapter discusses how sterility arises from differences in reproductive systems and the inheritance of characters from both parent-forms in hybrids and mongrels.
Characters to notice
- Kölreuter
Cited as a conscientious observer who made the rule of sterility universal, but ranked fertile forms as varieties.
- Gärtner
Cited as a careful experimenter who also made the rule of sterility universal, but whose methods (castration, seclusion) may have impaired fertility.
- Charles Darwin
Author of the chapter, presenting his argument that sterility of crosses is not due to natural selection.
- Mr. B. D. Walsh
Mentioned in the context of observations on hybrid sterility (implied by reference to Mr. Hewitt's observations on pheasants and fowls).
Key passages
“The view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion.”
Most naturalists believe that species are made sterile when they cross, to keep them from mixing.
Darwin introduces the prevailing view he will challenge.
“The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection.”
The inability of different species to produce fertile offspring when first crossed, and the infertility of their hybrid young, did not evolve because it was beneficial.
Key argument that sterility is an incidental byproduct, not an adaptation.
“It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins.”
Sterility between species varies continuously, and the fertility of pure species is easily influenced, making it hard to draw a clear line between fertility and sterility.
Darwin emphasizes the gradation of sterility, undermining the idea of a sharp species boundary.