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I hope that some of your readers will respond to Mr. Westwood’s wish, and give any information which they may possess on the permanence of cross-bred plants and animals.² Will Mr. Westwood be so good as to give a reference to any account of the variability of the Swedish Turnip?³ I did not even know that it was reputed to be a cross-bred production. I am aware that this is supposed to be the case with some Turnips; but I have searched in vain for any authentic history of their origin. No one, I believe, doubts that cross-bred productions tend to revert in various degrees to either parent for many generations; some say for a dozen, others for a score or even more generations. But cannot breeders adduce some cases of crossed breeds of sheep and pigs (such as the Shropshire or Oxford sheep, or Lord Harborough’s pigs) which are now true?⁴ With respect to the Cottagers’ Kale, I was so much surprised at the accounts of its trueness that I procured seed from the raisers; but in my soil the plants were far from presenting a uniform appearance.⁵ In addition to the tendency to reversion to either parent form, it is almost universally asserted that cross-bred productions are highly variable, and often display characters not observed in either parent. I do not wish to dispute this common belief, but I suspect it would puzzle any one to adduce satisfactory cases; and certainly Gärtner has advanced a mass of evidence on the opposite side.⁶ I am not at all surprised at Mr. Westwood demurring to the belief that occasionally crossing the strain is advantageous or necessary with productions in a state of nature. The subject is only just alluded to in my volume on the “Origin of Species.”⁷ I do not pretend that I can prove the truth of the doctrine; but I feel sure that many important facts and arguments can be adduced in its favour. The ill effects of close inter-breeding between the nearest relations, especially if exposed to the same conditions of life, would be, I believe, the same under Nature as under domestication,—namely, some degree of sterility and weakness of constitution. Variability arises from quite independent causes, and is to a certain extent counteracted in its early stages by the free crossing of the individuals of the same species. Mr. Westwood misunderstands me if he supposes that it is my opinion that the Ibis, for instance, keeps true to its kind “by occasional crosses with individuals of the same species which have not sprung from the same grandfather or great-grandfather.”⁸ I only believe that if individuals of the Ibis did vary, such crosses would tend to keep the species true; and further, if the young from a single pair increased so slowly that they all continued to inhabit the same small district, and if brothers and sisters often united during successive generations, then that the Ibis would rapidly deteriorate in fertility and constitution. Mr. Westwood advances the hive-bee, as probably a case of constant intercrossing. Andrew Knight, however, who specially attended to this point, has published his belief (Whether founded on sufficient evidence I will not pretend to say) that the queen-bee commonly unites with a drone from another community.⁹

Charles Darwin, Down, Bromley, Kent. ¹⁰