TRANSACTIONS OF SECTION D. 6^5 



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

 endeavoured to show how the blighting intluence of free intercrossing might he 

 overcome by physiological selection, Komaues, like Huxley, believing several 

 varieties might bo 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 intercrossing. 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 belore 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 who so fully realised the need of isola- 

 tion, 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 ofispring 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 to 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 oti'spriug the image of herself in make, 

 colour, and tumperament to whole-coloured bay Arab and Shetland ponies. This 

 pony instead of being swamped invariably 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. AVhat is true of the Equidie 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 Gallo- 

 ways. The wolf is 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 e.xperiment with a grey- doe, the grand- 

 daughter 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, and two grey 

 ofispring. Similar results are obtained with plants; hybrid orchids, e.y,, some- 

 times reproduce all the characters of one of the parents. 



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

 environment, are not only surticiently prepotent to escape being swamped by other 

 varieties, but are also, like the spotted rabbit, able to hand on the prepotency 

 almost unimpaired to a majoritj^ of their descendants, progressive development 

 along a definite line will be possible. But of even more importance than pre- 

 potency is what for want of a better name may be known as exclusive inheritance. 

 Itecently 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 pro- 

 duced birds with only sixteen feathers in the tail when mated with an ordinary 

 dovecot pigeon. A still more striking example of exclusive inheritance we have 

 in the crow family. The carrion crow and the hooded crow are so unlike in 

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



