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. 1 Among plants, we have in 
North America green and yellow-fruited plums not affected by 
a disease that attacked the purple-fruited varieties. Yellow- 
Heshed peaches suffer more from disease than white-fleshed 
kinds. In Mauritius, Avhite 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 kinds. 2 
These curious and inexplicable correlations of colour with 
constitutional peculiarities, both in animals and plants, render 
it probable that the correlation of colour with infertility, 
which has been detected in several cases in plants, may also 
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 species by 
natural selection has been greatly exaggerated, because the 
1 In the Medico-Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has 
adduced some curious physiological facts bearing on the presence or absence 
of white colours in the higher animals. He states that a dark pigment in the 
olfactory region of the nostrils is essential to perfect smell, and that this 
pigment is rarely deficient except when the whole animal is pure white, and 
the creature is then almost without smell or taste. He observes that there is 
no proof that, in any of the cases given above, the black animals actually eat 
the poisonous root or plant ; and that the facts are readily understood if the 
senses of smell and taste are dependent on a pigment which is absent in the 
white animals, who therefore eat what those gifted with normal senses avoid. 
This explanation however hardly seems to cover the facts. We cannot sup¬ 
pose that almost all the sheep in the world (which are mostly white) are 
without smell or taste. The cutaneous disease on the white patches of hair 
bn horses, the special liability of white terriers to distemper, of white chickens 
to the gapes, and of silkworms which produce yellow silk to the fungus, are 
not explained by it. The analogous facts in plants also indicate a real con¬ 
stitutional relation with colour, not an affection of the sense of smell and 
taste only. 
2 For all these facts, see Animals and Tlants under Domestication, vol. iL 
pp. 335-338. 
