September 12, 1901] 



NA TURE 



485 



Intercrossing and Interbreeding as Causes of Variation. 



The belief was once common amongst naturalists that vari- 

 ability was wholly due to crossing, and at the present day 

 naturalists and breeders alike agree that intercrossing is a 

 potent cause of variability, and are unanimous in regarding in- 

 terbreeding as an equally potent means of checking variability. 

 The opinion is also general that intercrossing has a swamping 

 influence ; that having brought forth new forms it forthwith 

 proceeds to destroy them. Darwin, when discussing reversion, 

 points out that intercrossing often speedily leads to almost 

 complete reversion to a long-lost ancestor, i.e., to the loss of 

 recently acquired and the reappearance of long lost characters 

 " .\nimals and Plants," vol. i. p. 22). When, however, he 

 comes to deal with variability, he states that "crossing, like 

 any other change in the conditions of life, seems to be an ele- 

 ment, probably a potent one, in causing variability (ibid., vol. 

 ii. p. 254), the offspring of the first generation being generally 

 uniform, but those subsequently produced displaying an almost 

 infinite diversity of character. As to the influence of inbreeding 

 lie says, "close interbreeding, if not carried to an injurious ex- 

 treme, far from causing variability, tends to fix the character of 

 each breed (ibid., vol. ii. p. 251). 



These statements may be quoted in support of the very 

 common belief that intercrossing is both a potent cause of varia- 

 tion and of reversion ; that it produces new varieties one 

 moment and swamps them the next. Whether intercrossing 

 may be regarded as the immediate cause of variation or of 

 reversion (it can hardly be both) depends on what is implied 

 by variation. Obviously variation may be either progressive or 

 retrogressive, i.e., the oftspring may differ from their parents in 

 having quite new characters or in presenting ancestral charac- 

 ters, or in being characterised by traits neither new nor old, due 

 to new combinations of characters already recognised as 

 belonging to the variety or species. When intercrossing 

 results in the restoration of old characters, we have reversion 

 or retrogressive variation ; when to new combinations of 

 already existing characters like new combinations in a kaleido- 

 scope, we have new variations of a non-progressive kind, 

 almost always characterised by more or less reversion ; when, 

 however, intercrossing results in the characters of one variety 

 being engrafted on another, or to the appearance of characters 

 quite new to the species, we have progressive variation. Judg- 

 ing from the results I have obtained, intercrossing of two dis- 

 tinct varieties results, as a rule, in the loss of the more striking 

 characters of both parents, i.e. in more or less marked rever- 

 sion, the extent of the loss generally depending on the difter- 

 ence between the forms crossed. For example, if an owl pigeon 

 is crossed with a pigeon known among fanciers as an archangel, 

 nondescript birds are obtained, which may at once, with a white 

 fantail, give birds almost identical with a blue-rock — the 

 common ancestor of all our breeds of pigeons. Intercrossing, 

 on the other hand, rarely leads to the blending in one indi- 

 vidual of the unaltered characters of two or more varieties, 

 and it never results in the appearance of characters 

 absolutely new to the species. In a word, the imme- 

 diate result of intercrossing distinct varieties is, as a 

 rule, more or less marked reversion. But though inter- 

 crossing usually results in retrogressive variation, it is indirectly 

 an extremely potent cause of progressive variation. This is 

 due to the fact (better realised by botanists than zoologists) that 

 cross-bred offspring (first crosses) are (unless the parents have 

 been enfeebled by interbreeding) endowed with an unusual 

 amount of vigour, i.e., intercrossing is of supreme importance, 

 not only because it leads to the co-mingling of germ-plasms 

 having different tendencies, but also and perhaps chiefly be- 

 cause of its rejuvenating influence. The importance of this re- 

 juvenation is usually at once evident if intercrossing is imme- 

 diately followed by interbreeding. The interbreeding of closely 

 related forms generally reduces the vigour, and, as Darwin 

 points out, "far from causing variability, tends to fix the 

 character of each breed" (" Animals and Plants," vol. ii. p. 251) ; 

 but the intercrossing of first crosses (or of highly vigorous in- 

 dividuals closely related in either the direct or the collateral 

 line) without appreciably weakening the constitution, often re- 

 sults in offspring displaying, to use Darwin's words, "an almost 

 infinite diversity of character '' (ibid., vol. ii. p. 256). The epi- 

 demics of variation, so often the outcome of interbreeding first 

 or at least vigorous recently produced crosses, are apparently 

 partly due to the union of individuals having a similar tendency 

 checking reversion, and partly to the vigour acquired by recent 



NO. 1663, VOL. 64] 



intercrossing. This much may be inferred from the fact, that 

 when interbreeding is persisted in the variability dwindles as 

 the vigour ebbs. 



Breeders agree with Darwin that first crosses are generally 

 uniform, and that the subsequent offspring usually vary im- 

 mensely ; yet neither breeders nor naturalists seem to have 

 clearly realised that interbreeding at the right moment is the 

 direct cause of variation, while intercrossing is, except in very 

 rare cases, at the most an indirect cause of variation. 



It may be here said that it is impossible to over-estimate the 

 importance of vigour in studying variation. Without vigour no 

 race or breed can maintain its position ; without renewed vigour 

 it is hardly likely to develop new characters. The new vigour, 

 as already explained, may be obtained by intercrossing ; but it 

 may also be acquired, especially in plants, by a change of 

 surroundings accompanied by a plentiful supply of suitable 

 food. 



With rigid selection the gradual loss of vigour may escape 

 notice, but when selection is suspended rapid deterioration 

 (from the fancier's standpoint) is the inevitable result. If, e.g., 

 a number of pigeons, good specimens of a distinct breed, are 

 isolated and left unmolested for a few years, they rapidly degen- 

 erate, i.e., they lose their show points(be they peaks, frills, ruffs, 

 or metallic tints) and reassume the more fixed ancestral char- 

 acters. If, however, the less characteristic birds are eliminated, 

 and high-class birds are from time to time introduced from 

 another loft, the vigour and the distinctive traits are indefinitely 

 preserved. 



If the age and condition of the soma and the state of ripeness 

 of the germ-cells are potent factors, and especially if vigour 

 counts for much, the difficulties of breeders become intelligible, 

 and the unlikeliness of intercrossing being a direct cause of 

 variation all the more evident. The most that can be expected 

 from intercrossing is the engrafting on one breed the characters 

 of another. Even this rarely happens, and is only possible 

 when the two breeds are somewhat allied. It is impossible, 

 e.g., to unite m one individual all the points of a fantail and a 

 pouter, or of a fantail and a jacobin ; but given healthy, 

 vigorous birds, the points of an owl may be engrafted on a barb. 

 Or to take another example, the black ears, feet, &c. , of a 

 Himalaya rabbit may be combined with the characteristic form, 

 long hair and habits of an Angora. It may be impossible to 

 predict what will happen when intercrossing is resorted to, but 

 if pure-bred members of a distinct variety are experimented with 

 — and it is useless working with either plants or animals of un- 

 known origin — characters not already present in one of the 

 varieties need not be looked for. 



But while interbreeding at the right moment may be a cause 

 of progressive variation, at other times it leads to what is per- 

 haps best described as degeneration. When, i;.^., very young 

 members of the same brood or litter, or unhealthy, closely 

 related individuals, or quite mature and apparently vigorous but 

 for several generations closely related animals are interbred, the 

 offspring frequently differ from their parents. They are often 

 delicate and highly sensitive, and unable to survive unless pro- 

 vided with highly nutritious food ; and though they mature 

 numerous germ-cells they rear but few offspring, and, what is 

 still more striking, they are sometimes either white or all but 

 devoid of pigment. Ofl'spring thus characterised, especially 

 when white or nearly white in colour, e.g., nearly white 

 pheasants, partridges, .ind woodcock, white specimens of the 

 brown hare, white squirrels, &c., are sometimes regarded as 

 distinct varieties, but when the departure from the normal 

 colour, &c., is the result of close inbreeding it is best regarded 

 as degeneration. 



In the spring of 1900 I crossed a quarter-wild grey doe rabbit 

 with a closely inbred black-and-white buck. The young 

 obtained varied considerably in colour : to one of her olfspring 

 coloured like the sire, the grey doe produced a second litter, all 

 but one decidedly lighter in colour than the sire. Two of the 

 darker members of this litter produced almost white young, and 

 to one of them the original grey doe has recently produced a 

 light-coloured litter consisting of two pure white specimens, two 

 with only a narrow dorsal band, two fawn-coloured and one 

 black. Close interbreeding with goats and pigeons jnelds 

 similar results. Birds on small remote Pacific islands are some- 

 times marked with irregularly disposed white patches. These 

 pie-bald birds, like light- coloured pheasants, cream-coloured 

 partridges and dun-coloured rooks, may also be the victims of 

 close inbreeding. 



