Independently of colour, other trifling differences are sometimes found to be of importance to plants under cultivation, and would be of paramount importance if they had to fight their own battle and to struggle with many competitors. The thin-shelled peas, called _pois sans parchemin_, are attacked by birds[558] much more than common peas. On the other hand, the purple-podded pea, which has a hard shell, escaped the attacks of tomtits (_Parus major_) in my garden far better than any other kind. The thin-shelled walnut likewise suffers greatly from the tomtit.[559] These same birds have been observed to pass over and thus favour the filbert, destroying only the other kinds of nuts which grew in the same orchard.[560] Certain varieties of the pear have soft bark, and these suffer severely from boring wood-beetles; whilst other varieties are known to resist their attacks much better.[561] In North America the smoothness, or absence of down on the fruit, makes a great difference in the attacks of the weevil, "which is the uncompromising foe of all smooth stone-fruits;" and the cultivator "has the frequent mortification of seeing nearly all, or indeed often the whole crop, fall from the trees when half or two-thirds grown." Hence the nectarine suffers more than the peach. A particular variety of the Morello cherry, raised in North America, is without any assignable cause more liable to be injured by this same insect than other cherry-trees.[562] From some unknown cause, the Winter Majetin apple enjoys the great advantage of not being infested by the coccus. On the other hand, a particular case has been recorded in which aphides confined themselves to the Winter Nelis pear, and touched no other kind in an extensive orchard.[563] The existence of minute glands on the leaves of peaches, nectarines, and apricots, would not be esteemed by botanists as a character of the least importance, for they are present or absent in closely related sub-varieties, descended from the same parent-tree; yet there is good evidence[564] that the {232} absence of glands leads to mildew, which is highly injurious to these trees.