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Darwin Correspondence Project

From Francis Galton   19 December 1875

42 Rutland Gate

Dec 19/75

My dear Darwin

The explanation of what you propose, does not seem to me in any way different on my theory, to what it wd. be on any theory of organic units.1 It would be this:—

Let us deal with a single quality, for clearness of explanation, & suppose that in some particular plant or animal & in some particular structure, the hybrid between white & black forms was exactly intermediate, viz gray,—thenceforward for ever. Then a bit of the tinted structure under the microscope would have a form which might be drawn as in a diagram, as follows:—



Whereas in the hybrid, it would be either that some cells were white and others black, & nearly the same proportion of each, thus:—


giving on the whole when less highly magnified, a uniform gray tint,—or else this:—


in which each cell had a uniform gray tint.

In (1) we see that each cell has been an organic unit (quoad2 colour)  In other words, the structural unit is identical with the organic unit

In (2), the structural unit would not be an organic unit but it would be an organic molecule. It would have been due to the development, not of one gemmule but of a group of gemmules,3 in which the black & white species would, on statistical grounds, be equally numerous (as by the hypothesis, they were equipotent.)

The larger the number of gemmules in each organic molecule, the more uniform will the tint of grayish be in the different units of structure. It has been an old idea of mine, not yet discarded & not yet worked out, that the number of units in each molecule may admit of being discovered by noting the relative number of cases of each grade of deviation from the mean grayness. If there were 2 gemmules only, each of which might be either white or black, then in a large number of cases one quarter would always be quite white, one quarter quite black, & one half would be gray. If there were 3 molecules, we should have 4 grades of colour (1 quite white, 3 light gray, 3 dark gray, 1 quite black) & so on according to the successive lines of “Pascal’s triangle”.4 This way of looking at the matter would perhaps shew (a) whether the number in each given species of molecule was constant & (b), if so, what those numbers were.

Ever very faithfully yrs | Francis Galton


CD had challenged Galton’s theory of heredity by asking him to explain how hybrids that were intermediate in character between their parents produced buds that reproduced this intermediate character (see letter to Francis Galton, 18 December [1875] and n. 3).
Quoad (Latin): ‘as regards, with respect to’ (OED).
CD hypothesised that every unit that made up an organism threw off minute granules or gemmules, which were transmitted from the parents to the offspring (Variation 2: 374).
Pascal’s triangle is a triangular pattern of numbers with each being the sum of the two numbers above it, except for the side edges, which are all number one. It is used to calculate the number of possible combinations, and the probability of any combination.


OED: The Oxford English dictionary. Being a corrected re-issue with an introduction, supplement and bibliography of a new English dictionary. Edited by James A. H. Murray, et al. 12 vols. and supplement. Oxford: Clarendon Press. 1970. A supplement to the Oxford English dictionary. 4 vols. Edited by R. W. Burchfield. Oxford: Clarendon Press. 1972–86. The Oxford English dictionary. 2d edition. 20 vols. Prepared by J. A. Simpson and E. S. C. Weiner. Oxford: Clarendon Press. 1989. Oxford English dictionary additional series. 3 vols. Edited by John Simpson et al. Oxford: Clarendon Press. 1993–7.

Variation: The variation of animals and plants under domestication. By Charles Darwin. 2 vols. London: John Murray. 1868.


Outlines in simple form the statistical distribution of inherited characteristics in a theory of "organic units".

Letter details

Letter no.
Francis Galton
Charles Robert Darwin
Sent from
London, Rutland Gate, 42
Source of text
DAR 105: A92–3
Physical description
ALS 4pp

Please cite as

Darwin Correspondence Project, “Letter no. 10309,” accessed on 5 July 2022,

Also published in The Correspondence of Charles Darwin, vol. 23