The Origin of Species · Chapter
Origin and Causes of the Sterility of First Crosses and of Hybrids explained
Darwin examines the sterility observed when different species are crossed, arguing that it cannot be explained by natural selection alone.
What happens
Darwin examines the sterility observed when different species are crossed, arguing that it cannot be explained by natural selection alone. He presents evidence from reciprocal crosses, graded sterility, and the parallel effects of unnatural conditions on reproduction, concluding that sterility arises from the disturbance of the reproductive system when two distinct organizations are blended or when conditions of life are altered.
Themes in this chapter
Natural Selection and the Struggle for Existence
Darwin argues that natural selection cannot explain the sterility of first crosses and hybrids, as it would not favor individuals that are more sterile.
Variation and Inheritance
The chapter explores how variations in fertility arise and are inherited, and how crossing affects the reproductive system.
Characters to notice
- Charles Darwin
Author and primary voice, presenting arguments against natural selection as the cause of hybrid sterility.
- Gärtner
Cited for experiments showing graded sterility in crosses within genera.
- Kölreuter
Cited alongside Gärtner for evidence of a series from reduced seed production to complete sterility.
- Herbert Spencer
Referenced for the principle that life depends on the action and reaction of forces tending toward equilibrium.
Key passages
“At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility...”
Darwin initially considered that sterility in crosses could be gradually enhanced by natural selection, but later rejected this idea.
Key argument against natural selection as the cause of hybrid sterility.
“Both Gartner and Kolreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed...”
Experiments by Gärtner and Kölreuter show a continuous range from reduced seed set to complete sterility in crosses.
Empirical evidence for graded sterility.
“Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected in a very similar manner.”
Both environmental change and hybridization disturb the reproductive system in analogous ways.
Parallel between sterility from altered conditions and from crossing.