676 REPORT— 1901. 



be two varieties of the same species. The carrion crow is black all over, but ill 

 the hooded crow the breast aud 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 Ijotli 

 varieties occurring in the same nest. Similar exclusiveness occurs amongst 

 mammals. 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 rabbits ; 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 exclusiveness we have in the Ancon or ' Otter ' 

 sheep common in New England at the end of the eighteenth century. This breed, 

 -which was characterised 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 iu 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 stili, the Ancou-like crosses, generation after generation, 

 were as exclusive as their crooked-legged ancestor. 



Another familiar example of exclusiveness we have iu 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 surround- 

 ings as on whether they are sufhcientlj' prepotent and exclusive to escape being 

 swamped by intercrossing. This way of accounting for progress in one or more 

 directions may prove as inadequate as the one suggested 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 meeting of the Association) ' the advan- 

 tageously varied bridegroom at tlie 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 very well afford to maintain a number of budding organs, it 

 helps us to understand how luminous, electric, and certain other structures were 

 nursed up to the point when they began to count iu the struggle for existence. 



Doubtful Causes of Variation. 



Having indicated how maturity of the soma and of the germ-cells, and how 

 bodily welfare and interbreeding may act as causes of variation, and also how 

 swamping of the new variations may be checked, I shall now refer to certain 

 supposed causes of variation. 



Maternal Impressions. 



I may begin with the widespread belief that the offspring are capable of beiug 

 influenced in form, colour, and temperament by maternal impressions — the belief 

 we associate with the skilful shepherd who peeled wands and stuck them up before 

 the fulsome ewes. MuUer,' more than half a century ago, conclusively argued 

 against the belief iu maternal impressions, but the belief still prevails. I know of 



1 



Elements of Physiology, vol. ii. p. 1405. 



