while kept away from others by effective defenses. (Halicta feeds on the deadly nightshade, which is poisonous to most animals.) I have no doubt that the butterflies, whose wonderful instinct leads them to deposit their eggs upon the plants best adapted to nourish their young, are aided in distinguishing between species and often warned off of the wrong ones by such spots and markings; for they well know how to put two and two together.
The coloring matters of plants are closely connected chemically with the aromatic group of substances (Vines). Naturally, then, odor sometimes takes the place and does the work of color. The presence of distasteful oils, resins, etc., is announced by the odor of some plants no less effectively than that of obnoxious tannin, stinging raphides, or bitter alkaloids, by the colors of others. The most brilliant flowers are not the sweetest; both qualities are not ordinarily needed (though they may exist together, as in the hard-fighting plants of the Mediterranean region); 14·6 per cent of the white flowers are odorous, only 3·2 per cent of the red. And odor, like color, may be at the same time attractive and repellent—a phenomenon probably much more common than we imagine.
Ten of the thirty species mentioned by Kerner as not eaten by herbivorous mammals are either aromatic or strong-scented. Pasture mints and field onions are avoided by cattle. What hungriest mammal would relish a meal of the skunk-cabbage or the spotted arum? Yet flies are attracted to both. If the offensive odor which some beetles and caterpillars emit when handled is a warning, surely that of these plants says more plainly than words, "Nemo me impune lacessit." Herbivora seem even to be repelled by the sweet fragrance of wintergreen, lily of the valley, violets, and of some orchids, which attracts to them their insect friends.
Again, the same animal may be simultaneously attracted and warned by a color. Dr. Ogle found the white and blue varieties of a species of monk's-hood growing in the same district of Switzerland. Almost every opened corolla of the white variety was perforated; none of the blue. The flowers are dependent for pollination upon bees; the perforated white variety therefore produced no seed, and this form was rare; but the blue, entered legitimately and abundantly pollinated, common. So the importance of the warning is evident. The blue corolla invites guests to the feast of honey, but at the same time proclaims that it is for their interest to get it lawfully. The bees understand that unpleasant properties of some kind are associated with the color of this form.