NA TURE 



[September 12, 1901 



The Swamping Effects of Intercrossing. 

 The question " Are new varieties liable to be swamped by 

 intercrossing?" is perhaps the most important now pressing for 

 an answer from biologists. What would happen, for example, 

 if specimens of all the different breeds of cattle were set free and 

 left unmolested on a large area ? Would 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 different 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 (" Origin of Species," 

 p. 104). Unfortunately Darwin nosvhere indicates how he sup- 

 posed new varieties escape being swamped by intercrossing. 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 varieties which were more or less in- 

 fertile " (" Life of Professor Huxley," p. 170). Later 

 Moritz \Vagner and others pointed out the important 

 part physical isolation had played in the origin of species ; and 

 later still Romanes endeavoured to show how the blighting 

 influence of free intercrossing might be overcome by physio- 

 logical selection, Romanes, like Huxley, believing several 

 varieties might be evolved in the same area if more or less 

 mutually infertile. Evidence of the importance of physical 

 isolation is plentiful enough ; but neither has experimental nor 

 selective breeding proved that physiological isolation has 

 been instrumental in arresting the swamping effects of inter- 

 crossing. Hence, according to Huxley and others, the 

 foundation of Darwin's doctrine of natural selection must still 

 be regarded as insecure. Is intersterility the only possible 

 means by which new varieties can be saved from premature 

 extinction, from being destroyed before they have a chance of 

 proving their fitness to survive ? In other words, are barriers 

 as essential among wild as among domestic animals? It does 

 not seem to have occurred to the biologists w^ho so fully realised 

 the need of isolation that the old varieties instead of swamping 

 might be swamped by the new, and that several varieties might 

 sometimes be sufficiently exclusive to flourish and eventually 

 give rise to a like number of species in the same area. If on 

 an island two new varieties of sheep appeared sufficiently 

 vigorous, or, as we say, sufficiently prepotent, to swamp all 

 the other varieties — as the ill-favoured lean kine did eat up 

 the fat ones — and yet so exclusive that their cross-bred off- 

 spring invariably belonged to the one new variety or the 

 other, for their preservation fences and other barriers would 

 be superfluous. 



Is there any evidence that by prepotency the swamping of 

 new varieties is sometimes checked, and that by exclusive 

 inheritance two or more varieties, though mutually fertile, may 

 persist in the same area, occasionally intercrossing with each 

 other, but neither giving up nor taking from each other any of 

 their distinctive characters? I have in my possession a 

 skewbald Iceland pony that produces richly striped hybrids to 

 a zebra, but skewbald offspring the image of herself in make, 

 colour, and temperament to whole-coloured bay Arab and 

 Shetland ponies. This pony instead of being swamped invari- 

 ably swamps older breeds. A number of prepotent skewbald 

 ponies, wherever placed, would (especially with the help of 

 preferential mating) in all probability soon give rise to a distinct 

 race such as once existed in the East. What is true of the 

 Equidte is equally true of other groups. Black hornless 

 Galloway bulls are often so prepotent that their offspring with 

 long-horned brightly-coloured Highland heifers readily pass for 

 pure-bred Galloways. The wolf is so prepotent over the dog, 

 as the wild rabbit, rat, and mouse are prepotent over their tame 

 relatives. As an instance of prepotency in rabbits, I may give 

 the results of an interbreeding experiment with a grey doe, the 

 granddaughter of a wild rabbit, and an inbred buck richly 

 spotted like a Dalmatian hound. Of six young in the first litter 

 three were like the sire. To one of her sons, the grey doe next 

 produced eight young, all richly spotted, and subsequently to one 

 of her spotted grandsons she produced two spotted, two white, 



NO. 1663, VOL. 64] 



and two grey offspring. Similar results are obtained with 

 plants ; hjJbrid orchids, e.g., sometimes reproduce all the char- 

 acters of one of the parents. 



It need hardly be insisted on that if new varieties, well 

 adapted for their environment, are not only sufficiently pre- 

 potent to escape being swamped by other varieties, but are also, 

 like the spotted rabbit, able to hand on the prepotency almost 

 unimpaired to a majority of their descendants, progressive 

 development along a definite line will be possible. But of even 

 more importance than prepotency is what for want of a better 

 name may be known as exclusive inheritance. Recently a 

 vigorous mature Indian blue-rock pigeon mated with an inbred 

 and equally mature fantail, hatched and reared two birds, one 

 exactly like a blue-rock, but with fourteen instead of twelve 

 tail feathers ; the other characterised by all the points of a high- 

 class fantail, the tail feathers being thirty in number — two fewer 

 than in the fantail parent, but eighteen more than in the blue- 

 rock parent. In this case the blue-rock was the exclusive bird, 

 the fantail having previously produced birds with only sixteen 

 feathers in the tail when mated with an ordinary dovecot 

 pigeon. A still more striking example of exclusive inheri- 

 tance we have in the crow family. The carrion crow and 

 the hpoded crow are so unlike in colour that they were. 

 long regarded as two distinct species ; now they are said to 

 be two varieties of the same species. The carrion crow is black 

 all over, but in the hooded crow the breast and back are grey. 

 These two crows cross freely (but for this they would probably 

 still rank as distinct species) ; but in the crossbred young there 

 is never any blending — they are either black or grey, usually 

 both varieties occurring in the same nest. Similar exclusiveness 

 occurs amongst mamcnals. When distinct varieties of cats are 

 crossed, some of the young usually resemble one breed, some the 

 other, and the distinctions may persist for several generations. 

 A white crossed with a tabby-coloured Persian cat produced a 

 pair of white and a pair of tabby-coloured young ; the two white 

 cats when interbred also produced two white and two tabby- 

 coloured individuals. I find cats are far more exclusive than rab- 

 bits ; perhaps it is partly for this reason we have so many species 

 and varieties of wild cats, so few species and varieties of wild 

 rabbits. Another very striking instance of e.%clusiveness we have 

 in the Ancon or "Otter" sheep comnum in New England at 

 the end of the eighteenth century. This breed, which was char- 

 acterised by short crooked legs and a long back like a turnspit dog, 

 descended from a ram-lamb born in Massachusetts in 1791. The 

 offspring of this "sport" were never intermediate in their 

 characters ; they were either like the original Ancon ram or like 

 the breeds, .some thirteen in number, with which he was mated. 

 Frequently in the case of twins one was otter-like, the other an 

 ordinary lamb. More remarkable still, the Ancon-like crosses, 

 generation after generation, were as exclusive as their crooked- 

 legged ancestor. 



Another familiar example of exclusiveness we have in the 

 peppered moth, a dark variety of which in a few years swamped 

 the older light variety throughout a considerable part of England, 

 and is now extending its range on the Continent. It thus 

 appears that when a new variety is sufficiently prepotent, instead 

 of being swamped it may actually swamp the old-established 

 variety ; and that when two or more varieties are sufficiently 

 exclusive they may flourish side by side, and eventually give rise 

 to two or more distinct species. 



Prepotency may hence be said to supplement and complete the 

 work of the environment The environment seems to be mainly 

 concerned in eliminating the unfit ; whether any of the survivors 

 persist depends not so much on their surroundings as on 

 whether they are sufficiently prepotent and exclu.sive to escape 

 being swamped by intercrossing. This way of accounting for 

 progress in one or more directions may prove as inadequate as 

 the one sviggested by isolationists, but it has the merit of being 

 more easily tested by experiment. It not only gets rid of the 

 swamping bugbear, but makes it matter of indifference whether 

 (to quote from the President's address at the last Oxford meet- 

 ing of the Association) " the advantageously varied bridegroom 

 at the one end of the wood meets the bride, who, by a happy 

 contingency, had been advantageously varied in the same 

 direction, and at the same time, at the other end of the wood." 

 Further, as a highly prepotent vigorous variety can well afford 

 to maintain a number of budding organs, it helps us to under- 

 stand how luminous, electric, and certain other structures were 

 nursed up to the point when they began to count in the struggle 

 for existence. 



