skip to content

Darwin Correspondence Project

From John Wright   11 June 1868

Yeldersley House, | Derby.

June 11 | 1868.

Dear Sir

With regard to your question namely “on the preference of female dogs horses or other animals for particular males etc”.. I can only say that there is no doubt whatever that occasionally a preference exists, & that to the general rule that a female will at certain seasons admit any male of her kind to sexual intercourse, there are exceptions.1 This is certainly the case, occasionally in breeding horses, a mare will sometimes evince particular dislike to a certain stallion, & will lash out at him without mercy, but will admit another immediately after, & I have known a case where the mare had to be blindfolded & then cheated by another stallion being brought up to her for an instant while she was covered by the one she had taken the dislike to, but this dislike without any apparent reason whatever—2

Bitches too will sometimes take a dislike to a certain dog. & dogs will take no notice of certain bitches, but in both cases without any apparent reason whatever. some assert that it is often the case in crossing two very distinct breeds of dogs but I have not found it oftener in such cases. I had a mastif bitch that I wanted to put to one of my deerhounds, but he would take no notice of her whatever, but another deerhound I put her with immediately afterwards, immediately accomplished the desired act.3

In the case of breeding mules there is I believe not unfrequently some difficulty in getting the stallion donkey to cover the mare tho: there again often there is none—

With regard to the proportional number of male & female births with animals (with birds I have had little experience tho I am much inclined to think the rule to be the same) my opinion is that the numbers of each sex are nearly equal the female slightly predominating, but to produce a satisfactory & fair result you must take the young of a certain number of females in one year & then the young of a certain number of different females in another, & let the cases be taken from a period extending over not less than 8 or 10 years. I say this because I am inclined to think that some years are for some reasons or other more propitious towards the birth of one sex than another, & I also say that examples must be taken from a large number of females because some females decidedly have very often indeed a preponderance of one sex over another in their progeny. For-instance I have an Arab mare & she has had 7 foals & every time by a different horse except in one instance, but every foal she has had has been a filly.

Bitches will sometimes have a very large proponderance of one sex over the other & sometimes all of one sex & even in very large litters, eight dogs perhaps & one or two bitches & the exactly reverse, but in the case of bitches I have never found the proponderance of one sex over another in any one litter of any bitch a criterion to go by that there will be the same proponderance in her next litter but on the contrary it is often exactly the reverse. I am rather inclined to think that the number of sexes proponderating the one over the other in bitches is due to the state of the female at the time of impregnation, there is much indeed that may be said upon bitches at the time of their heat, but as regards number of produce I have generally found that the oftener the bitch is warded the smaller the litter is likely to be; but that is entering into another subject with which I dare say you are well acquainted, & I fear that the little I have now said upon the two other questions will give you no information that you have not already.

I will conclude by sending you the number of male & female calves during the last 7 years in a small herd of Brittany cows that I have, the females as you will see preponderating, & the females slightly preponderating will perhaps generally be the case but not to any great extent where a large number are taken—perhaps one in six.

I remain | Dear Sir | Yrs. very Sincerely | John Wright

I shall be very happy at any time to hear from you & to endeavour to throw light on any queries.


1862 14 cows produce 5 male 9 female
1863 17 cows " 7 " 10 "
1864 10 cows " 5 " 5 "
1865 14 cows " 5 " 9 "
1866 14 cows " 5 " 9 "
1867 12 cows " 8 " 4 "
1868 7 cows " 4 " 3 "
39 49

CD annotations

1.1 With … & that to 1.3] crossed blue crayon
1.3 the general] after opening square bracket, blue crayon
1.5 mare will sometimes 1.6 ] underl blue crayon
1.7 have known a case] underl blue crayon
1.10 apparent reason] underl blue crayon
2.1 Bitches … whatever. 2.2 ] enclosed in square brackets blue crayon
2.1 Bitches … act. 2.7] crossed ink
2.4 mastif bitch] underl blue crayon
2.5 deerhounds,] underlined blue crayon
2.5 he would] underl blue crayon
3.1 In … examples 4.9] crossed blue crayon
4.10 some females] underl blue crayon
4.10 often indeed a preponderance] underl blue crayon
4.11 one sex over another] underl blue crayon
4.12 every time … filly. 4.13] scored blue crayon
5.3 but in the case of bitches] underl blue crayon
5.6 I am … six. 6.5] crossed blue crayon
Top of leaf containing paras 4–6: ‘John Wright’ ink; ‘Proportion of sexes’ ink, partially circled, ink
Top of letter: ‘Preference of Individual Dogs & Bitches & Mares | On individual animals producing more ♀s or ♂s.’ blue crayon
Top of enclosure: ‘Brittany cows from John Wright of Yeldersley House near [interl] Derby’ ink


CD’s letter to Wright has not been found. In May 1868, CD had been writing to animal-breeders enquiring, amongst other things, about female preference for particular mates, and the proportion of male to female births (see, for example, letter from George Cupples, 26 May 1868). Wright had been mentioned to CD as a possible correspondent by George Cupples, in his letter of 26 May 1868.
CD cited Wright for this information in Descent 2: 272.
CD cited Wright for this information in Descent 2: 271–2.


Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.


Preference of females for particular males certainly exists occasionally.

On the proportion of males to females in horses and in dogs.

Letter details

Letter no.
John Wright
Charles Robert Darwin
Sent from
Source of text
DAR 83: 163–4, DAR 85: B38, DAR 86: A95–6
Physical description
8pp †

Please cite as

Darwin Correspondence Project, “Letter no. 6240,” accessed on 26 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 16