white terriers suffer more than terriers
of any other colour from the fatal Distemper.[541] In North America
plum-trees are liable to a disease which Downing[542] believes is not
caused by insects; the kinds bearing purple fruit are most affected, "and
we have never known the green or yellow fruited varieties infected {228}
until the other sorts had first become filled with the knots." On the other
hand, peaches in North America suffer much from a disease called the
_yellows_, which seems to be peculiar to that continent, and "more than
nine-tenths of the victims, when the disease first appeared, were the
yellow-fleshed peaches. The white-fleshed kinds are much more rarely
attacked; in some parts of the country never." In Mauritius, the white
sugar-canes have of late years been so severely attacked by a disease, that
many planters have been compelled to give up growing this variety (although
fresh plants were imported from China for trial), and cultivate only red
canes.[543] Now, if these plants had been forced to struggle with other
competing plants and enemies, there cannot be a doubt that the colour of
the flesh or skin of the fruit, unimportant as these characters are
considered, would have rigorously determined their existence.

Liability to the attacks of parasites is also connected with colour. It
appears that white chickens are certainly more subject than dark-coloured
chickens to the _gapes_, which is caused by a parasitic worm in the
trachea.[544] On the other hand, experience has shown that in France the
caterpillars which produce white cocoons resist the deadly fungus better
than those producing yellow cocoons.[545] Analogous facts have been
observed with plants: a new and beautiful white onion, imported from
France, though planted close to other kinds, was alone attacked by a
parasitic fungus.[546] White verbenas are especially liable to mildew.[547]
Near Malaga, during an early period of the vine-disease, the green sorts
suffered most; "and red and black grapes, even when interwoven with the
sick plants, suffered not at all." In France whole groups of varieties were
comparatively free, and others, such as the Chasselas, did not afford a
single fortunate exception; but I do not know whether any correlation
between colour and liability to disease was here observed.[548] In a former
chapter it was shown how curiously liable one variety of the strawberry is
to mildew.

{229}

It is certain that insects regul