Let us consider first of all the menace to plants from the larger herbivores, from elephants and cattle down to the hare and the roe-deer; we find that many plants are protected by poisons, which develop in the sap of their stems, leaves, roots, and fruits. The juicy and beautifully leaved Belladonna (_Atropa belladonna_) is never touched by roe-deer, stags, or other herbivores, and the same is true of the thorn-apple (_Datura stramonium_), the henbane (_Hyoscyamus niger_), the spotted hemlock (_Conium maculatum_), the danewort of our woods (_Sambucus ebulus_), and many others; they all contain a poison. Like the unpalatable butterflies, these unpalatable plants are also furnished with a warning sign of their undesirability, namely, a disagreeable odour, perceptible even by man, which scares off animals from touching them. The development of this through natural selection presents no very serious difficulty. But, strangely enough, there are not a few poisonous plants in which we, at least, are unable to detect any such warning sign. Among these are the blue aconite (_Aconitum_), the black hellebore (_Helleborus niger_), the meadow-saffron (_Colchicum autumnale_), species of Gentian, of spurge (_Euphorbia_), and others. Yet these are avoided by deer, roe-deer, chamois, hares, and marmots, and our cattle, horses, and sheep also usually leave them untouched. A case has, however, been reported from the valley of the Aur, on the lower Rhine, which seems to contradict this. On the rocky grass-slopes of the valley the poisonous hellebore (_Helleborus viridis_) grows in great abundance, and the sheep of that region, which were wont to graze on the slopes, avoided these plants. But some sheep from another part were imported into the valley, and these ate the hellebore, with the result that many died. If these poisonous plants, then, were furnished with a warning sign such as a disagreeable odour, not perceptible to us, we should have to assume that the imported sheep had a less acute sense of smell than the others, which is not impossible in domesticated animals. If there were no such warning sign, then it must have been not an instinct but a continuous _tradition_ which prevented the native sheep from touching the inedible plants. A more naïve interpretation of nature than that of our day would have regarded the fragrant ethereal oils developed in the seeds of many plants, as in those of fennel, cummin, and other Umbelliferous plants, as a peculiarity designed for the use and profit of man. But these ethereal substances are obviously a means of protection against the depredations of seed-eating birds, for a sparrow which was allowed to eat three or four seeds of cummin died very soon afterwards.