Chap. XXI. NATURAL SELECTION. 213 



to run together in man and the lower animals. Thus white 

 terriers suffer more than those of any other colour from the 

 fatal distemper.^ In North America plum-trees are liable to 

 a disease which Downing^ 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 

 " until the other sorts had first become filled with the knots." 

 On the other hand, peaches in Xorth 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 Mauri- 

 tius, 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.^ 

 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. "White chickens are certainly more subject than dark- 

 coloured chickens to the gapes, which is caused by a parasitic 

 worm in the trachea.^" 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.^^ 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.^^ White verbenas are especially liable 

 to mildew.^^ Near Malaga, during an early period of the vine- 



' Youatt on the Dog, p. 232. 379. 



* 'The Fruit-ti-ees of America,' " Quatrefages, ' Maladies Actuelles 

 1845, p. 270 : for peaches, p. 466. du Ver a Soie,' 1859, pp. 12, 214. 



* *Proc. Royal Soc. of Arts and '^ ' Gardener's Chronicle,' 1851, p. 

 Sciences of Mauritius,' 1852, p. 595. 



czxzv. '^ * Journal of Horticulture,' 1862, 



" 'Gardener's Chronicle,' 1856, p. p. 476. 



