[Enclosure]

Down— Beckenham, | Kent

7 March 1878

My dear Farrer,

M^r. Torbitt’s plan of resisting the potato disease seems to me by far the best that has ever been suggested. It consists, as you know from his printed letter to the Chancellor of the Exchequer, of raising a vast number of seedlings from cross-fertilized parents, exposing them to infection, destroying all which suffer, saving those which resist best, & repeating the process in successive seminal generations.⁶ For instance, the plants raised from seeds this present spring will be carefully examined in the autumn, and the tubers of those which seem most free from the disease will alone be saved and planted next spring. Their flowers will be cross-fertilized in the summer; their seeds saved, again sown, and so onwards.

My belief in the probability of good results from this process of continued elimination rests on the fact of all characters occasionally varying. It is known that the potato varies in its liability to the attacks of the fungus: some of the kidney varieties have suffered so much that they are no longer cultivated: on the other hand it can hardly be doubted that some kinds resist better than others; for if this had not been the case it is incredible that experienced growers should have sent a ton of tubers with the risk of paying half the expenses of their cultivation, to be tested by the Agricultural Society; though none were found fungus-proof.⁷ A few cases may be alluded to, showing that varieties occasionally arise which are exempt from the attacks of various enemies. Certain varieties of the American vine resist Phylloxera far better than the European vine;⁸ certain varieties of the apple, both in England and in South Australia, have been observed to resist completely the attacks of Coccus: certain varieties of the peach resist mildew much better than others; and I could give several other analogous cases.⁹ Sir J. Hooker informs me that the Liberian coffee withstands the White Fly which destroys the ordinary kind; and in accordance with the facts here alluded to, he has urged the cultivators of coffee and tea in India not to rely on any single variety of these plants.¹⁰ On the other hand, as M^r. Carruthers’ remarks in a memorandum on the present Subject addressed to M^r. Caird, no variety of wheat has ever arisen which can resist rust.¹¹ With respect to the cross-fertilization of distinct seedling plants, it has been ascertained that their offspring inherit much more vigorous constitutions, so that they increase much more rapidly by suckers &^c, & are generally more prolific in seed, than are seedlings from self-fertilized parents. 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 seldom get crossed by our native insects; & some varieties are even absolutely sterile unless fertilised with pollen from a distinct kind.¹²

From the above considerations it appears to me that M^r Torbitt has a good chance of success if he follows out his plan for some more generations. M^r Carruthers, who has had great experience in the trials made by the Agricultural Society, objects in the memorandum before referred to, that there are no grounds for believing that M^r Torbitt’s varieties have greater power of resisting the disease than those which have been tried & have failed. This may be the case, but as far as is known to me, no one has hitherto gone on systematically selecting seedlings from parents which have been exposed to infection & which have been proved to be in some degree free from the disease, during several successive generations, aided by cross-fertilisation. It should be borne in mind that when an organism has once begun to vary in any one direction it probably will go on varying in the same direction for a length of time; for it is on this principle that all our domesticated animals & plants have been improved. As M^r Torbitt has already been at work for three years, & has now got a very large stock of cross-fertilised seeds, saved from plants which exhibited some, or as he believes a very considerable degree of immunity from the disease, it would be a great misfortune if he were prevented from continuing the work from the want of a small grant of money. I know from analogous experiments what unremitting care & patience are necessary for work of this kind; & it is in the highest degree improbable that any one else could be found who would carry on the work efficiently & gratuitously. Should a variety be raised which resists completely or almost completely the disease, its value would soon be recognized in the surrounding district & would thence spread rapidly throughout the Kingdom.

My dear Farrer | Yours sincerely | Charles Darwin