The Variation of Animals and Plants Under Domestication, Volume II (of 2). Darwin Charles
aggregated differences which distinguish domestic races and species from each other, not being liable to this peculiar form of transmission, we may conclude that it is in some way connected with the sudden appearance of the characters in question.
On the Modification of old Races and the Formation of new Races by Crossing.– We have hitherto chiefly considered the effects of crossing in giving uniformity of character; we must now look to an opposite result. There can be no doubt that crossing, with the aid of rigorous selection during several generations, has been a potent means in modifying old races, and in forming new ones. Lord Orford crossed his famous stud of greyhounds once with the bulldog, which breed was chosen from being deficient in scenting powers, and from having what was wanted, courage and perseverance. In the course of six or seven generations all traces of the external form of the bulldog were eliminated, but courage and perseverance remained. Certain pointers have been crossed, as I hear from the Rev. W. D. Fox, with the foxhound, to give them dash and speed. Certain strains of Dorking fowls have had a slight infusion of Game blood; and I have known a great fancier who on a single occasion crossed his turbit-pigeons with barbs, for the sake of gaining greater breadth of beak.
In the foregoing cases breeds have been crossed once, for the sake of modifying some particular character; but with most of the improved races of the pig, which now breed true, there have been repeated crosses, – for instance, the improved Essex owes its excellence to repeated crosses with the Neapolitan, together probably with some infusion of Chinese blood.200 So with our British sheep: almost all the races, except the Southdown, have been largely crossed; "this, in fact, has been the history of our principal breeds."201 To give an example, the "Oxfordshire Downs" now rank as an established breed.202 They were produced about the year 1830 by crossing "Hampshire and in some instances Southdown ewes with Cotswold rams: " now the Hampshire ram was itself produced by repeated crosses between the native Hampshire sheep and Southdowns; and the long-woolled Cotswold were improved by crosses with the Leicester, which latter again is believed to have been a cross between several long-woolled sheep. Mr. Spooner, after considering the various cases which have been carefully recorded, concludes "that from a judicious pairing of cross-bred animals it is practicable to establish a new breed." On the Continent the history of several crossed races of cattle and of other animals has been well ascertained. To give one instance: the King of Wurtemberg, after twenty-five years' careful breeding, that is after six or seven generations, made a new breed of cattle from a cross between a Dutch and Swiss breed, combined with other breeds.203 The Sebright bantam, which breeds as true as any other kind of fowl, was formed about sixty years ago by a complicated cross.204 Dark Brahmas, which are believed by some fanciers to constitute a distinct species, were undoubtedly formed205 in the United States, within a recent period, by a cross between Chittagongs and Cochins. With plants I believe there is little doubt that some kinds of turnips, now extensively cultivated, are crossed races; and the history of a variety of wheat which was raised from two very distinct varieties, and which after six years' culture presented an even sample, has been recorded on good authority.206
Until quite lately, cautious and experienced breeders, though not averse to a single infusion of foreign blood, were almost universally convinced that the attempt to establish a new race, intermediate between two widely distinct races, was hopeless: "they clung with superstitious tenacity to the doctrine of purity of blood, believing it to be the ark in which alone true safety could be found."207 Nor was this conviction unreasonable: when two distinct races are crossed, the offspring of the first generation are generally nearly uniform in character; but even this sometimes fails to be the case, especially with crossed dogs and fowls, the young of which from the first are sometimes much diversified. As cross-bred animals are generally of large size and vigorous, they have been raised in great numbers for immediate consumption. But for breeding they are found to be utterly useless; for though they may be themselves uniform in character, when paired together they yield during many generations offspring astonishingly diversified. The breeder is driven to despair, and concludes that he will never form an intermediate race. But from the cases already given, and from others which have been recorded, it appears that patience alone is necessary; as Mr. Spooner remarks, "nature opposes no barrier to successful admixture; in the course of time, by the aid of selection and careful weeding, it is practicable to establish a new breed." After six or seven generations the hoped-for result will in most cases be obtained; but even then an occasional reversion, or failure to keep true, may be expected. The attempt, however, will assuredly fail if the conditions of life be decidedly unfavourable to the characters of either parent-breed.208
Although the grandchildren and succeeding generations of cross-bred animals are generally variable in an extreme degree, some curious exceptions to the rule have been observed, both with crossed races and species. Thus Boitard and Corbié209 assert that from a Pouter and a Runt "a Cavalier will appear, which we have classed amongst pigeons of pure race, because it transmits all its qualities to its posterity." The editor of the 'Poultry Chronicle'210 bred some bluish fowls from a black Spanish cock and a Malay hen; and these remained true to colour "generation after generation." The Himalayan breed of rabbits was certainly formed by crossing two sub-varieties of the silver-grey rabbit; although it suddenly assumed its present character, which differs much from that of either parent-breed, yet it has ever since been easily and truly propagated. I crossed some Labrador and Penguin ducks, and recrossed the mongrels with Penguins; afterwards, most of the ducks reared during three generations were nearly uniform in character, being brown with a white crescentic mark on the lower part of the breast, and with some white spots at the base of the beak; so that by the aid of a little selection a new breed might easily have been formed. In regard to crossed varieties of plants, Mr. Beaton remarks211 that "Melville's extraordinary cross between the Scotch kale and an early cabbage is as true and genuine as any on record;" but in this case no doubt selection was practised. Gärtner212 has given five cases of hybrids, in which the progeny kept constant; and hybrids between Dianthus armeria and deltoides remained true and uniform to the tenth generation. Dr. Herbert likewise showed me a hybrid from two species of Loasa which from its first production had kept constant during several generations.
We have seen in the earlier chapters, that some of our domesticated animals, such as dogs, cattle, pigs, &c., are almost certainly descended from more than one species, or wild race, if any one prefers to apply this latter term to forms which were enabled to keep distinct in a state of nature. Hence the crossing of aboriginally distinct species probably came into play at an early period in the formation of our present races. From Rütimeyer's observations there can be little doubt that this occurred with cattle; but in most cases some one of the forms which were allowed to cross freely, will, it is probable, have absorbed and obliterated the others. For it is not likely that semi-civilized men would have taken the necessary pains to modify by selection their commingled, crossed, and fluctuating stock. Nevertheless, those animals which were best adapted to their conditions of life would have survived through natural selection; and by this means crossing will often have indirectly aided in the formation of primeval domesticated breeds.
Within recent times, as far as animals are concerned, the crossing of distinct species has done little or nothing in the formation or modification of our races. It is not yet known whether the species of silk-moth which have been recently crossed in France will yield permanent races. In the fourth chapter I alluded with some hesitation to the statement that a new breed, between the hare and rabbit, called leporides, had been formed in France, and was found capable of propagating itself; but it is now positively affirmed213 that this is an error. With plants which can be multiplied by buds and cuttings, hybridisation has done wonders, as with many kinds of Roses, Rhododendrons, Pelargoniums,
Richardson, 'Pigs,' 1847, pp. 37, 42; S. Sidney's edition of 'Youatt on the Pig,' 1860, p. 3.
'Gardener's Chronicle,' 1857, pp. 649, 652.
'Bulletin de la Soc. d'Acclimat.,' 1862, tom. ix. p. 463.
'Poultry Chronicle,' vol. ii., 1854, p. 36.
'The Poultry Book,' by W. B. Tegetmeier, 1866, p. 58.
'Gardener's Chronicle,' 1852, p. 765.
Spooner, in 'Journal Royal Agricult. Soc.,' vol. xx., part ii.
'Les Pigeons,' p. 37.
Vol. i., 1854, p. 101.
'Cottage Gardener,' 1856, p. 110.
'Bastarderzeugung,' s. 553.
Dr. Pigeaux, in 'Bull. Soc. d'Acclimat.,' tom. iii., July 1866, as quoted in 'Annals and Mag. of Nat. Hist.,' 1867, vol. xx. p. 75.