674 EEPORT— 1901. 



intercrossing is tlic eugraff ing on ouo breed of the cbaracters of another. Kveu this 

 rarely happens, and is only possible when the two breeds are somewhat allied. It 

 is impossible, e.g., to nnite in 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, &e., 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 iinkuown 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 perhaps best described as degeneration. 

 When, e.ij., 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 ofl'spring frequently difler 

 from their parents. They are often delicate and highly sensitive, and unable to 

 survive unless provided 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. Offspring thus 

 characterised, especially when white or nearly white in colour, e.y., nearly white 

 pheasants, partridges, and 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 better to regard 

 it as a form of degeneration. 



In the spring of 1900 I cro&.^ed 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 ollspring 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-Avhite 

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

 black. Close interbreeding with goats and pigeons yields similar results. Birds 

 on small remote I'acilic islands are sometimes marked with irregularly disposed 

 white patches. These pie-bald birds, like light-coloured ])heasants, cream-coloured 

 ]iartridgL's. and dun-coloured rooks, may also be the victims of close inbreeding. 



Tha Hvcmninwj Effects of Intercrossiwj. 



The question ' Are new varieties liable to be swamped by intercrossing P ' is 

 perhaps the most important now pressing for an answer from biologists. V/hat 

 would happen, for example, if specimens of all the ditlerent breeds of cattle were 

 set free and left unmolested on a large area ? AVould they some centuries hence 

 be represented by several breeds or by one ? Many would answer this question by 

 saying that unless some of them in course of time were isolated by mountains, 

 deserts, or other physical barriers, they would eventually through intercrossing 

 give rise to a single breed. To this question Darwin would, I think, have given a 

 somewhat diffeient answer, for, while admitting ' that isolation is of considerable 

 importance in the production of new species,' he was, on the whole, ' inclined to 

 believe that largeness of area is of more importance.' ' Unfortunately Darwin 

 nowhere indicates how he supposed new varieties escape being swamped by inter- 

 crossing. His silence on this important point is difficult to explain, for during his 

 lifetime the influence of intercrossing in checking progress, except in one direction, 

 was often enough insisted on. Huxley tells us that in his earliest criticisms of the 

 ' Origin ' ' he ventured to point out that its logical foundation was insecure so long 

 as experiments in selective breeding had not produced vaiieties which were more 

 or less infertile.' - Later Moritz AVagner and others pointed out the important 



' Origin of Species, p. 10-1. ^ Life of Professor Huxley, p, 170. 



