[Enclosure]

(Statement with respect to M^r Torbitt’s experiments on the Potato.)

M^r Torbitt of Belfast asked me in 1875, whether I thought favourably of a plan which he had already commenced for combating the potato disease.⁵ His plan was to raise many thousand seedlings, to destroy all which were tainted, to preserve the varieties which seemed to resist best, to raise fresh seedlings from them, & so onwards for successive generations. In fact he intended to apply to the potato the principle of Selection which has yielded such wonderful results with animals in the hands of British agriculturists. This plan appeared to me hopeful, because with plants long cultivated every or almost every character becomes more or less variable; & there seemed no extreme improbability in a variety arising which from the structure of its tissues or nature of its juices might successfully resist the fungus. We have analogous cases in certain species & varieties of the American vine having naturally arisen which can resist the Phylloxera; in certain varieties of the apple not being attacked by the Coccus both in England & Australia; & in some varieties of the Peach resisting mildew better than others. Sir J. Hooker informs me that the Liberian coffee withstands the White fly better than the other kinds, & he has urged cultivators of Coffee & Tea not to rely on any single variety.⁶ We see something of the same kind even with man, in different individuals being more or less liable to certain infectious germs; & this liability is said to run in families. It may, however, be justly opposed that as the fungus of the potato attacks at least one other distinct species of the genus, namely the Tomato, it is very unlikely that a fungus-proof variety of the potato should arise. On the other hand the probability of such a variety arising is greatly strengthened by the notorious fact that the already existing varieties differ much in their liability to be attacked; & this has held good in a conspicuous manner with the many new varieties raised by M^r Torbitt.

In my first letter to M^r. Torbitt, I suggested to him the advantage of cross-fertilising his seedlings in each generation, as I knew by experience that this would largely increase their fertility & constitutional vigour; the latter being shown by such seedlings resisting sudden changes of temperature & obtaining food from soil clogged with the roots of other plants.⁷ M^r Torbitt has acted on this suggestion, with, as he believes favourable results.)

M^r Torbitt has worked on a large scale, & this alone offered a fair chance of success; for he has raised each year about 5000 seedlings, & latterly has had ten acres of land under cultivation for his varieties. He has raised two successive generations from selected & cross-fertilised plants, & this year intends to raise a third generation. He informs me that the cross-fertilised seedlings of 1877 were “unquestionably more free from the disease than those raised in 1875.”⁸ The difference between the same two sets of plants in 1879 was so great, that it could be perceived at the distance of nearly a mile; as the leaves of the 1875 plants were of a paler colour & had not grown sufficiently to conceal the ground. The leaves of the twice-crossed young seedlings of 1879 suffered much from the disease; & this is a bad feature in the case, but as far as M^r Torbitt can judge the tubers of some of the varieties are quite sound. Looking to all the varieties which he has raised, he states positively that many of them “are so prolific & so free from the disease, that they leave, after separating the few diseased tubers, a larger crop of sound tubers than the common old varieties of sound & diseased tubers taken together”.⁹

I append a statement by M^r Torbitt dated Nov^r. 1879, & printed for private circulation. He has sent me several letters & testimonials, all more or less favourable, with respect to the earlier varieties; & 8 additional ones are dated 1878 & 1879.¹⁰ In some of these it is stated that certain vars. are quite free of the disease   He endeavoured last autumn to get two well-known agriculturists to report on his crops; but they could not spare the time. He hopes to succeed in getting authenticated reports this autumn.

Finally M^r Torbitt estimates as nearly as he can, that his experiments have cost him, since their commencement, nearly 1000^£. He has given away new varieties largely, & has realised only a few pounds by the sale of superfluous tubers. He has received no pecuniary aid (before the present spring) except 100^£ which I sent him in March 1878, & I here mention this as evidence that to the best of my judgment his plan is hopeful.¹¹ No one who has not carried on analogous experiments can be aware of the time & labour requisite for selecting, keeping separate & labelling numerous varieties,—for cross-fertilising the flowers, saving the seed, raising seedlings & potting them in a greenhouse &c. M^r Torbitt cannot afford to continue his experiments any longer without some aid; & it appears to me that it would be a great misfortune should the labour already taken be almost thrown away. According to all analogy the chance of raising sound varieties will be stronger in each successive selected generation. It is more especially desirable that the experiments should be continued, because there is reason to believe that lately raised varieties resist the disease better than those which have been long cultivated; so that until some of M^r Torbitts new varieties have been tested for a few more years, it is impossible to rely on them with confidence.

Charles Darwin

March. 24^th 1880 | Down Beckenham Kent

P.S. It may be well to add that I am not personally acquainted with M^r Torbitt, but from our correspondence I have formed a high opinion of his probity.—

My letter←Mr Caird