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Darwin Correspondence Project

From G. B. Murdoch   10 March 1871

March 10th. 1871—

Charles Darwin Esqre.


In your ‘Descent of Man’ in treating of the external differences between males and females of the same variety have you attached sufficient importance to the different amount and kind of energy expended by them in reproduction?1

In the female as a rule the vigour required for reproduction is brought into existence as it is required—spread equably over a considerable period and produced to just the extent, or but little in excess of the extent, needed for the gradual development and rearing of the young.

In the male the vigour produced is vastly in excess of that required for the single act, or series of single acts he performs in direct connection with reproduction. He has, particularly in those males which have a breeding season, to accumulate vigour, just as an athlete does before an exhibition of his strength; and this is not done as in a spring, wound up to be released at once; it is only gained by exercise; it is only by the expenditure of vigour that vigour is gained.

In the breeding male that vigour is expended in singing playing and fighting and possibly in increased size of muscles, limbs, & body and particularly in increased developments of his more readily developed parts, his fins, feathers, hair or horns.

No doubt this working up of the male to a high pitch of vigour has been developed itself, but none the less may it be a cause of the secondary developments of his attributes.

If so sexual selection as far as relates to the choice & caprice of the female is less the cause of these developments than an influence in directing them, their direction being modified as well, and their extent regulated by natural selection, by the food habitat and general circumstances of the animal

Thus what you evidently consider a difficulty viz: the high sense of beauty required in females of a very low rank in the scale of being—and their extraordinary discrimination, if differences were always as slight as they are now—is avoided—2 The necessity of an excess in the number of males is also avoided though of course this excess would add to the rapidity of development and tend to check deviations from the direction which the development had taken.

Does it not also simplify the connection of varieties with a common original, to put their divergence upon the surplus vigour of the male, modified by different circumstances of life, rather than upon the differences of female taste.

The surplus breeding vigour of the male taken as the cause of his secondary developments, directly connects them with those sexual attributes which are transmitted to the male through the female; and these transmitted attributes are capable of division into the transmitted result of ancestral development which would be apparent even in castrated animals; and into the transmitted tendency to develop the individual vigour in a certain direction which would be wholly or partly cut off by castration.

The cases in which the female is more conspicuous than the male is also explicable on the supposition that she has more surplus breeding vigour than the male, if there are no facts to bar that supposition.

As each individual receives the ancestral development and transmits it unimpaired or somewhat increased the tendency is to a constant increase in the male—subject of course to the retarding influence of the nonprogressing female—or, if she progresses also limited by the law of natural selection.

Again among those animals which have lost or not acquired a breeding season we may expect less marked developments particularly less marked annual developments, perhaps also among those whose habits of life require a large constant expenditure of energy—

To conclude (see ‘Descent of Man’ Vol I p. 318)—3

Does it not follow that a being must consume much more matter or force in its growth complicated structure or activity, than another being which produces ovas and embryos of large size & expends much energy in nurturing its young, so that these two beings may eat the same food and follow the same habits?

Is it wrong then to suppose that extra growth complicated structure and activity in one sex relative to the other sex do exist as escape valves for surplus vigour rather than to please lovers or fight rivals, though they may serve these purposes and be modified by them?—

I am, Sir, | Yours faithfully | George B. Murdoch | Civil Engineer

20 Buchanan St. | Glasgow.

CD annotations

6.1 If so … animal 6.4] double scored red crayon
Top of letter: ‘Mr Murdoch’ pencil


In Descent 1: 254–6, CD noted that some differences between males and females of the same species were related to different ‘habits of life’ but that he would limit his discussion to characteristics modified by sexual selection.
See Descent 1: 322–3.
In Descent 1: 318, CD discussed Herbert Spencer’s idea of the ratio between ‘individuation and genesis’ (energy needed for growth and reproduction).


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


Relation of surplus vigour of males to sexual selection.

Letter details

Letter no.
George Brown Murdoch
Charles Robert Darwin
Sent from
Source of text
DAR 90: 68–71
Physical description
7pp †

Please cite as

Darwin Correspondence Project, “Letter no. 7564,” accessed on 20 October 2019,

Also published in The Correspondence of Charles Darwin, vol. 19