“Darwin believed that the crossing of two types generally led to a blend, and that consequently bisexual reproduction tended to make a species uniform. He therefore had to postulate some cause constantly at work to keep up the inheritable variation within a species. He very naturally looked to the effects of differences of environment. …It was shown that Darwin had been wrong in supposing that variations due to environment were inheritable. Selection merely picked out the best available line from a given population, and would not, as Darwin had believed, give rise to an unlimited amount of change.”
Introduction, pp.14-20.
The Causes of Evolution (1932)
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J. B. S. Haldane 32
Geneticist and evolutionary biologist 1892–1964Related quotes

Source: On the Origin of Species (1859), chapter III: "Struggle For Existence", page 61 http://darwin-online.org.uk/content/frameset?pageseq=76&itemID=F373&viewtype=image
Context: Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.

Species and Varieties: Their Origin by Mutation (1904), The Open Court Publishing Company, Chicago, p. 5-6
"Double Trouble", pp. 38–40
The Panda's Thumb (1980)
Source: Images of Organization (1986), p. 39; As cited in as Vivien Martin -(2003) Leading change in health and social care. p. 157: About the organization as organism.

Source: The Geological Evidences of the Antiquity of Man (1863), Ch.21, p. 413

Ernst Mayr (1988) Toward a new philosophy of biology: observations of an evolutionist. p. 457
"The Tallest Tale", p. 312
Leonardo's Mountain of Clams and the Diet of Worms (1998)