It might indeed be supposed that extermination by these could hardly be an imminent danger, but if we think of the cockchafer blight, or of the destruction of whole woods by the caterpillar of the 'white nun,' or even of the destruction of several successive plantings of young salad plants which the snails often cause in our gardens, it cannot be doubted that all plants would be exterminated by insects and snails alone unless they were protected against them in some degree. We owe our detailed knowledge of the means by which plants protect themselves against the menace of the greedy and prolific snails to the beautiful investigations of Stahl, Professor of Botany in the University of Jena. In this case, too, both chemical and mechanical means are made use of. The minute quantity of tannic acid which is contained in the leaves of the clover prevents many snails from eating them, as, for instance, the garden snail (_Helix hortensis_). If the leaves be soaked so as to wash out the tannin the snail readily accepts them as food. It is true that the small, whitish field-slug (_Limax agrestis_) does not object to the presence of the tannin, and eats the fresh leaves of the clover; indeed, there is no such thing as absolute protection. In discussing the herbivorous mammals I have already mentioned that many trees and shrubs, mosses and ferns are effectively protected by the large amount of tannin they contain; this protection is effective also against snails, for all these plants are fairly free from their attacks; and the same is true of many other tannin-containing plants, species of saxifrage and sedum, the strawberry, many water-plants, like the pond-weeds (_Potamogeton_), the horn-nut (_Trapa_), the mare's tail (_Hippuris_). All these plants are only eaten by snails in case of necessity, or in the washed-out state. In other plants protection is gained by means of some acid, especially oxalic acid, like the wood-sorrel (_Oxalis acetosella_), the sorrel (_Rumex_), and the species of Begonia. When Stahl smeared slices of carrot, which is a favourite food of snails, with a weak (one per cent.) solution of oxalate of potassium, they were refused by the snails, and this is not surprising when we remember that even the external skin of the snail is very sensitive, and the mucous membrane of the mouth is not likely to be less so. Similarly, many plants develop ethereal oils in the hairs which cover them, as in the herb-Robert (_Geranium robertianum_). Even the almost omnivorous field-slug (_Limax agrestis_) does not touch this plant, and if it be placed upon it, escapes with all dispatch from the ethereal oil, which burns its naked skin, by covering itself with mucus and letting itself down to the ground by a thread. The mints (_Mentha_) and the dittany (_Dictamnus albus_) also produce such oils.