“The circumstance-adaptive law, operating upon the slight but continued natural disposition to sport in the progeny (seedling variety), does not preclude the supposed influence which volition or sensation may have over the configuration of the body. To examine into the disposition to sport in the progeny, even when there is only one parent, as in many vegetables, and to investigate how much variation is modified by the mind or nervous sensation of the parents, or of the living thing itself during its progress to maturity; how far it depends upon external circumstance, and how far on the will, irritability and muscular exertion, is open to examination and experiment. In the first place, we ought to investigate its dependency upon the preceding links of the particular chain of life, varieties being often merely types or approximations of former parentage; thence the variation of the family, as well as of the individual, must be embraced by our experiments.”

April 18, 1867.
Letters to Carl Nägeli

On naval timber and arboriculture (1831), Appendix F, part II

Source: The Geological Evidences of the Antiquity of Man (1863), Ch.21, p. 411-412
Context: The competition of races and species, observes Mr. Darwin, is always most severe between those which are most closely allied and which fill nearly the same place in the economy of nature. Hence, when the conditions of existence are modified, the original stock runs great risk of being superseded by some one of its modified offshoots. The new race or species may not be absolutely superior in the sum of its powers and endowment to the parent stock, and may even be more simple in structure and of a lower grade of intelligence, as well as of organisation, provided, on the whole, it happens to have some slight advantage over its rivals. Progression, therefore, is not a necessary accompaniment of variation and natural selection, though, when a higher organisation happens to be coincident with superior fitness to new conditions, the new species will have greater power and a greater chance of permanently maintaining and extending its ground.

Source: The Boy Crisis (2018), pp. 253

Source: Book 2, Chapter 1 “Whispering in Secret Rooms” (pp. 428-429), The Runestaff (1969)

The Universe - Sex in Space (2008)

sensation-complexes
Source: 20th century, The Analysis of Sensations (1902), p. 23, as quoted in Lenin as Philosopher: A Critical Examination of the Philosophical Basis of Leninism (1948) by Anton Pannekoek, p. 33

Hayek's Journey: The Mind of Friedrich Hayek (2003)