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Show 336 LAWS OF VARIATION. CHAP. xxv. southern parts of Europe, withstood the intense cold better than those with lighter complexions from the north; but no doubt such statements are liable to error. In the second chapter on Selection I have given several cases proving that with animals and plants differences in colour are correlated with constitutional differences, as shown by greater or less immunity from certain diseases, from the attacks of parasitic plants and animals, from burning by the sun, and from the action of certain poisons. When all the individuals of any one variety possess an immunity of this nature, we cannot feel sure that it stands in any sort of correlation with their colour ; but when several varieties of the same species, which are similarly coloured, are thus characterised, whilst other coloured varieties are not thus favoured, we must believe in the existence of a correlation of this kind. Thus in the United States purplefruited plums of many kinds are far more affected by a certain disease than green or yellow-fruited varieties. On the other hand, yellow-fleshed peaches of various kinds suffer from another disease much more than the white-fleshed varieties. In the Mauritius red sugar-canes are much less affected by a particular disease than the white canes. White onions and verbenas are the most liable to mildew; and in Spain the green-fruited grapes suffered from the vine-disease more than other coloured varieties. Dark-coloured pelargoniums and verbenas are more scorched by the sun than varieties of other colours. Red wheats are believed to be hardier than white; whilst red-flowered hyacinths were more injured during one particular winter in Holland than other coloured varieties. With animals, white terriers suffer most from the distemper, white chickens from a parasitic worm in their trachere, white pigs from scorching by the sun, and white cattle from flies; but the caterpillars of the silk-moth which yield white cocoons suffered in France less from the deadly parasitic fungus than those producing yellow silk. The cases of immunity from the action of certain vegetable poisons, in connexion with colour, are more interesting, and are at present wholly inexplicable. I have already given a remarkable instance, on the authority of Professor Wyman, of all the hogs, excepting those of a black colour, suffering severely in Virginia from eating the root of the Lachnanthes tinctoria. C HAP. XXV. CORRELATED VARIABILITY. 337 According to Spinola and others,3i buckwheat (Polygonum fagopyrum), when in flower, is highly injurious to white or whitespotted pigs, if they are exposed to the heat of the sun but is ~uite innocuous to black pigs. By two accounts, th: Hyper~ cum crispum in Sicily is poisonous to white sheep alone; th~1r heads swell, their wool falls off, and they often die ; but ~Ius plant, according to Leece, is poisonous only when it grows m swamps; nor is this improbable, as we know how readily the poisonous principle in plants is influenced by the conditions under which they grow. Three accounts have been published in Eastern Prussia of .white .and white-spotted horses being greatly injured b; eat1~g mildewed and honeydewed vetches ; every spot of skin bearmg white hairs Lecoming inflamed and gangrenous. The Rev. J. Rodwell informs me that his father turned out about fifteen cart-horses into a field of tares which in parts svvarrned with black aphides, and which no doubt were honeydewed, and probably mildewed; the horses, with two exceptions, were chesnuts and ba!s with white marks on their faces and pasterns, and the wh1te parts alone swelled and became angry scabs. 'l'he two bay horses with no white marks entirely escaped all InJury: In Guernsey, when horses eat fools' parsley (/Ethusa cynapzum) they are sometimes violently purged; and this plant "has a peculiar effect on the nose and lips, causing " deep cracks and ulcers, particularly on horses with white " muzzles." 38 With cattle, independently of the action of any poison, cases have been published by Youatt and Erdt of c~taneous diseases with much constitutional disturbance (in on~ msta~ce after exposure to a hot sun) affecting every single pomt whwh bore a white hair, but completely passing over other parts of the body. Similar cases have been observed with horses.39 We thus see that not only do those parts of the skin which bear white hair differ in a remarkable manner from those bearing 37 This fact and the followincr cases when not stated to the contr:ry, ar~ taken from a very curious paper by Prof. Heusinger, in 'W ochenschrift fUr Heilknnde,' May 1846, s. 277. VOL. II. 38 Mr. Mogford, in the ' Veterinarian , quoted in ' The Field,' Jan. 22 1861 p' 545. ' ' . 39 'Edinburgh Veterinary Journal,' Oct. 1860, p. 347. |