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Show 170 DARWINISM CHAP. distinctive marks, and they are, therefore, seldom crossed with these of another colour; :wd even when they are so crossed, no notice would be taken of any slight diminution of fertility, since this is liable to occnr from many causes. Vv e have also reason to believe that fertility has been increased by long domestication, in addition to the fact of the original stocks being ex?eptionally fertile; and no experiments have been made on the differently coloured varieties of wild animal . There arc, however, a number of very curious facts showing tha,t colour in animals, as in plants, is often correlated with constitutional differences of a remarkable kind, and as these have a close relation to the subject we are discussing, a brief summary of them will be here given. Con·elation of Colow· with Constitutional Peculia1·ities. The correlation of a white colour and blue eyes in male cats with deafness, and of the tortoise-shell marking with. tho female sex of the same animaJ, are two well-known hut most extraordinary cases. Equally remarkable is the fact, communicated to Darwin by Mr. Tegetmcicr, th<.Lt white, yellow, pale blue, or dun pigeons, of all breeds, have the young birds born naked, while iu all other colours they are well covered with down. Here we have a case in which colour seems of more physiological importance th<1n all the varied structnral differences between the varieties and breeds of pigeons. In Virginia there is a plant c<1lled the paint-root (Lachn<1nthes tinctoria), which, when eaten by pigs, colours their bones pink, and causes the hoofs of all but the black varieties to drop off; so that black pigs only can be kept in the district.l Buckwheat in flower is also said to be injurious to white pigs but not to black In the Tarcntino, black sheep are not injured by e<.Lting the Hypericum crispum- a specie:; of St. J ohn's-wort-which kills white sheep. vVhitc terrier~; suffer most from eli temper; white chickens from the gapes. \Vhite-haircd hor. cs or cattle are subject to cutaneous diseases from which the dark coloured arc free · while both in Thuringia and the \Vest Indies, it has been noticed tl~at white or pale coloured cattle are much more troubled by flies than arc those which arc brown or black The same law even extends 1 Orig'in of Species, sixth edition, p. 9. VII ON THE INFERTILITY OF CROSSES 171 to insects, for it is found that silkworms which produce white cocoons resist the fungus disease much better than do those which produce yellow cocoons.! Among plants, we have in North America green and yellow-fruited plums not affected by a disease that attacked the purple-fruited varieties. Yellowfleshed peaches suffer more from disease than white-fleshed kinds. In Mauritius, white sugar-canes were attacked by a disease from which the red canes were free. White onions and verbenas are most liable to mildew ; and red-flowered hyacinths were more injured by the cold during a severe winter in Holland than any other kin<.ls. 2 These curious and inexplicable correlations of colour with constitutional peculiarities, both in animals and plants, render it probable that the correlation of colonr with infertility, which has been detected in several cases in plants, may al o extend to animals in a state of nature ; and if so, the fact is of the highest importance as throwing light on the origin of the infertility of many allied species. This will be better understood after considering the facts which will be now described. The Isolation of Varieties by Selective Association. In the last chapter I have shown that the importance of geographical isolation for the formation of new spc ics by natural selection has been greatly exaggerated, because the 1 In the .ltledico-Chirwrgical Tmnsactions, vol. liii. (1870}, Dr. Ogle has aduuced son1e curious physiologieal facts bearing on the presence or absence of white colours in the higher animals. He states that a dark pi<Tineut in the o~factory_ region of th_e nostrils is essential to perfect smell, a~d that this p1gment IS r~rely deficient ex_cept when the whole animal is pure white, a111l the creature IS then almost Without smell or ta. te. He observes that tl1ere is no pro_of that, in any of the cases given above, the black auimals actually eat the p01 ·onous root or plant; antl that the facts are readily undcr:tootl if the senses of smell antl taste are dependent on a pigment whicl1 is abHent in tl1e white animalR, who therefore eat what those gifted with normal senses avoid. This explanation however hardly seem to cover the facts. We cannot supP? se that ahno:st all the sheep in the world (which are mo. tly white) are Without smell or taste. The cutaneous di:ease on the white patcl1es of hair on horses, the speciall~ability of white terriers to distemper, of white chickens to the gal?es, and. of Silkworms which produce yellow silk to the fungu s, are not explamed by It. The analogous facts in plants also indicate a real constitutional relation with colour, not an affection of the sense of smell and tate only. 2 l~or all these facts, see Animals and Plants under Domestication, vol. ii pp. 335-338. |