“Where natural selection slackens, new forms may arise which would not survive under more rigid competition”

— J. B. S. Haldane , book The Causes of Evolution

Source: The Causes of Evolution (1932), Ch. IV Natural Selection, pp. 104-106.
Context: Where natural selection slackens, new forms may arise which would not survive under more rigid competition, and many ultimately hardy combinations will thus have a chance of arising.... Thus the distinction between the principal mammalian orders seems to have arisen during an orgy of variation in the early Eocene which followed the doom of the great reptiles... Since that date mammalian evolution has been a slower affair, largely a progressive improvement of the types originally laid down in the Eocene.
Another possible mode of making rapid evolutionary jumps is by hybridisation.... hybridisation (where the hybrids are fertile) usually causes an epidemic of variation in the second generation which may include new and valuable types which could not have arisen within a species by slower evolution.

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J. B. S. Haldane 32

Geneticist and evolutionary biologist 1892–1964

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From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
"Introduction", page 5 http://darwin-online.org.uk/content/frameset?pageseq=20&itemID=F373&viewtype=image
On the Origin of Species (1859)