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Darwin Correspondence Project

From G. W. Child   16 February [1870]1

Elmhurst, | Gt. Missenden.

Feb 16

Dear Sir

I have again read your chapter on the good effects of crossing since I received your note of Jany 27—2 It appears to me that the questions with wh: that chaptr & also my Essay deal are 3.3

(1) Is it proved that any amount of close breeding alone will produce degeneracy   I think you admit that it is not at the beginning of your chapter, & this is all that I contend for—4 The evidence is conflicting, but it appears to me that in sound logic one positive instance in wh: it can be shewn that an extreme degree of close-breeding may exist without any discoverable degeneracy out weighs any number of cases in wh: degeneracy is found to exist in asmuch as the antecedents in all such cases are so complicated that it is impossible to shew that the close breeding per se is in cause of the deterioration

I think that in considering those cases in wh: breeds of domestic animals have been found to degenerate greatly when close-bred, sufficient attention has not always been paid to the absolute uniformity of external conditions (climate, locality, food &c) under which they have often been kept for many generations— The cases wh: you mention (p 125 &c) in wh: breeders have been able to go on with the same blood when kept at different & distant establishments seem to lend some force to this suggestion— We know that a mere change of diet or change of air will often produce a good effect & that if I mistake not on animals as well as on man.

(2) The positive evidence of improvement by crossing produces much more effect on my mind than the negative— But even with regard to this I am inclined to think that in the case of domestic animals breeders sometimes speak of what is merely an improvement with reference to their special purposes (fattening qualities mere size, the like) as if it were a real advance in the organization of the creature— And again I think that some of the above suggestions might go some way to account for the improvement when it really exists by suggesting that it is due rather to the two animals brought together to breed having lived under more or less widely differing circumstances than to their coming of different stocks—

Finally (3) with regard to the extension of the doctrine of the deleteriousness of close-breeding up to the case of the marriage of blood relations among mankind wh: is indeed the proper subject of my Essay published first in 1863.5 It seems to me that to maintain this view in the face of the Pedigrees in the herd book & such facts as you mention at p 115 is almost paradoxical.6 It is like saying that when a drug taken in a large dose is productive of no discoverable effects whatever it is highly probable that if taken in a small dose it might act as a dangerous poison— May I not fairly answer with the old logicians that de non apparentibus et de non existentibus eadem est ratio?7 This last point therefore is the only one on wh: my slight investigations of this subject leads me to a conclusion greatly different from that to which your incomparably deeper one has led you—

Pray excuse my troubling you with so long a letter & believe me dear Sir | faithfully yours | Gilbert. W. Child

C. Darwin Es &c &c


The year is established by the relationship between this letter and the letter from G. W. Child, 26 January [1870].
CD’s letter to Child has not been found. Child refers to chapter 17 of Variation (Variation 2: 114–44).
Child refers to Essay 1, Marriages of consanguinity, in his Essays on physiological subjects (Child 1869).
CD wrote, ‘The evil results from close interbreeding are difficult to detect, for they accumulate slowly, and differ much in degree with different species … several causes interfere with our detecting the evil—such as the deterioration being very gradual, and the difficulty of distinguishing between such direct evil and the inevitable augmentation of any morbid tendencies which may be latent or apparent in the related parents’ (Variation 2: 114, 115).
Child’s essay on marriages of consanguinity was first published in the Westminster Review 80 (1863): 88–109.
In Variation 2: 117–18, CD discussed the apparently advantageous effects of close breeding in cattle and sheep.
De non apparentibus et de non existentibus eadem est ratio: concerning the invisible and the non-existent, the argument is the same.


Child, Gilbert William. 1869. Essays on physiological subjects. 2d edition. London: Longmans, Green, and Co.

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


Criticises chapter on good effect of crossing in Variation: (1) does not accept that inbreeding alone results in degeneracy; (2) good effects of crossing exaggerated; (3) denies deleterious effects of close marriage in humans.

Letter details

Letter no.
Gilbert William Child
Charles Robert Darwin
Sent from
Gt Missenden
Source of text
DAR 161: 142
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 6617,” accessed on 6 March 2021,

Also published in The Correspondence of Charles Darwin, vol. 18