In Virginia there is a plant called the
paint-root (Lachnanthes 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.[58]
Buckwheat in flower is also said to be injurious to white pigs but not
to black. In the Tarentino, black sheep are not injured by eating the
Hypericum crispum--a species of St. John's-wort--which kills white
sheep. White terriers suffer most from distemper; white chickens from
the gapes. White-haired horses or cattle are subject to cutaneous
diseases from which the dark coloured are free; while, both in Thuringia
and the West Indies, it has been noticed that white or pale coloured
cattle are much more troubled by flies than are those which are brown or
black. The same law even extends 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.[59] Among plants, we
have in North America green and yellow-fruited plums not affected by a
disease that attacked the purple-fruited varieties.
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