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Darwin Correspondence Project

To T. H. Farrer   2 March 1878

Down Beckenham. [4 Bryanston Street, London.]

March 2. 1878.

My dear Farrer.

Mr. Torbitt’s plan of overcoming the potato disease seems to me by far the best which has ever been suggested. It consists as you know from his printed letter, of rearing a vast number of seedlings from cross fertilized parents exposing them to infection, ruthlessly destroying all that suffer, saving those which resist best & repeating the process in successive seminal generations—1 My belief in the probability of good results from this process rests on the fact of all characters whatever occasionally varying   It is known, for instance, that certain species & varieties of the vine resist phylloxera better than others—2 Andrew Knight found one variety or species of the apple which was not in the least attacked by Coccus, & another variety has been observed in S. Australia— Certain varieties of the peach resist mildew, & several other such cases could be given—3 Therefore there is no great improbability in a new variety of the potato arising which would resist the fungus completely or at least much better than any existing variety. With respect to the Cross fertilization of two distinct seedling plants, it has been ascertained that the offspring thus raised inherit much more vigorous constitutions & generally are more prolific than seedlings from self fertilized parents—4 It is also probable that cross fertilization would be especially valuable in the case of the potato, as there is reason to believe that the flowers are seldom crossed by our native insects; and some varieties are absolutely sterile unless fertilized with pollen from a distinct variety—5 There is some evidence that the good effects from a cross are transmitted for several generations   it would not therefore be necessary to cross fertilize the seedlings in each generation; though this would be desirable, as it is almost certain that a greater number of seeds would thus be obtained   It should be remembered that a cross between plants raised from the tubers of the same plant, though growing on distinct roots, does no more good than a cross between flowers on the same individual— Considering the whole subject, it appears to me that it would be a national misfortune if the cross fertilized seeds in Mr Torbitts possession produced by parents which have already shewn some power of resisting the disease are not utilized by the Government or some Public body and the process of selection continued during several more generations—

Should the Agricult: Soc:6 undertake the work Mr Torbitts knowledge gained by experience would be especially valuable; & an outline of his plan is given in his printed letter. It would be necessary that all the tubers produced by each plant should be collected separately, & carefully examined in each succeeding generation.

It would be advisable that some kind of potato eminently liable to the disease should be planted in considerable numbers near the seedlings so as to infect them.

Altogether the trial would be one requiring much care & extreme patience, as I know from experience with analogous work, & it may be feared that it would be difficult to find any one who would pursue the experiment with sufficient energy   It seems therefore to me highly desirable that Mr Torbitt should be aided with some small grant so as to continue the work himself.—

Judging from his reports, his efforts have already been crowned in so short a time with more success than could have been anticipated; & I think you will agree with me that anyone who raises a fungus-proof potato will be a public benefactor of no common kind.

My dear Farrer. | Yours sincerely. | Charles Darwin.


For the printed letter, see the enclosure to the letter from James Torbitt, 24 February 1878. CD had suggested that Torbitt send a copy to Farrer (see letter to James Torbitt, 26 February 1878).
Phylloxera (Daktulosphaira vitifoliae) is a small sap-sucking insect native to North America, accidentally introduced to Europe, where it devastated native grape-vines because it attacked the roots. In American vines, the insect usually only affected the leaves. For a contemporary account of the insect and the vine-infestation that spread rapidly across Europe in the 1860s and 1870s, see Riley 1874.
Thomas Andrew Knight described a cider apple that was not attacked by the ‘American apple-bug (Eriosoma mali …)’ (a synonym of Eriosoma langierum, woolly apple aphid; Knight 1826). CD had referred to a similarly resistant apple in Variation 1: 349, describing the insect as ‘mealy bug or coccus’. See also letter to J. D. Hooker, 28 [February 1878] and n. 5. Coccus viridis is a soft scale insect that is hosted by apples and other fruits and vegetables. Powdery mildew on peaches is caused by the fungus Sphaerotheca pannosa.
For a summary of CD’s research on the role of cross-fertilisation in producing more vigorous offspring over successive generations, see Cross and self fertilisation, pp. 285–91; Torbitt had referred to CD’s work in his printed letter (see enclosure to letter from James Torbitt, 24 February 1878).
In Variation 2: 169, CD had remarked that potatoes and other plants long propagated by tubers and bulbs often failed to flower or to produce fertile flowers.
The Royal Agricultural Society of England. See letter from T. H. Farrer, 28 February 1878.


Cross and self fertilisation: The effects of cross and self fertilisation in the vegetable kingdom. By Charles Darwin. London: John Murray. 1876.

Riley, Charles Valentine. 1874. The grape phylloxera Popular Science Monthly 5: 1–16.

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


Describes James Torbitt’s plan for producing disease-resistant potato varieties. [Letter is an earlier version of 11406.]

Letter details

Letter no.
Charles Robert Darwin
Thomas Henry Farrer, 1st Baron Farrer
Sent from
Source of text
DAR 144: 88
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 11389,” accessed on 14 April 2021,