Dear Sir,—

You ask my opinion as to whether the employment of a bull supposed to be scrofulous is consistent with the ultimate interest of the breeder.¹ As a general rule I should defer to the judgment of any one who had experience on such a point, supposing that he was not biassed by interest or prejudice. But in this particular instance we have such good evidence of the inheritance of constitutional diseases, such as scrofula, consumption, &c., that it seems to me very rash to breed from an animal thus tainted. In all probability a large majority of the offspring from a scrofulous bull, paired with a perfectly sound cow, would be to all appearance sound, but it can hardly be doubted that the evil would be latent in many of them, and ready to break out in subsequent generations.²

I will venture to add a few remarks on the general question of close interbreeding. Sexual reproduction is so essentially the same in plants and animals, that I think we may fairly apply conclusions drawn from the one kingdom to the other. From a long series of experiments on plants, given in my book On the Effects of Cross and Self-Fertilisation, the conclusion seems clear that there is no mysterious evil in the mere fact of the nearest relations breeding together; but that evil follows (independently of inherited disease or weakness) from the circumstance of near relations generally possessing a closely similar constitution. However little we may be able to explain the cause, the facts detailed by me show that the male and female sexual elements must be differentiated to a certain degree, in order to unite properly, and to give birth to a vigorous progeny.³ Such differentiation of the sexual elements follows from the parents and their ancestors having lived during some generations under different conditions of life.

The closest interbreeding does not seem to induce variability or a departure from the typical form of the race or family, but it causes loss of size, of constitutional vigour in resisting unfavourable influences, and often of fertility. On the other hand, a cross between plants of the same sub-variety, which have been grown during some generations under different conditions, increases to an extraordinary degree the size and vigour of the offspring.

Some kinds of plants bear self-fertilisation much better than others; nevertheless it has been proved that these profit greatly by a cross with a fresh stock. So it appears to be with animals, for Shorthorn cattle—perhaps all cattle—can withstand close interbreeding with very little injury; but if they could be crossed with a distinct stock without any loss of their excellent qualities, it would be a most surprising fact if the offspring did not also profit in a very high degree in constitutional vigour. If, therefore, any one chose to risk breeding from an animal which suffered from some inheritable disease or weakness, he would act wisely to look out, not merely for a perfectly sound animal of the other sex, but for one belonging to another strain which had been bred during several generations at a distant place, under as different conditions of soil, climate, &c., as possible, for in this case he might hope that the offspring, by having gained in constitutional vigour would be enabled to throw off the taint in their blood.—

Charles Darwin,

March 22.