The Evolution Theory, Vol. 1 of 2

by August Weismann

To sum up. We have seen that there is among plants an extraordinary diversity of protective adaptations, which secures them from extermination by the larger herbivores.

Since all useful contrivances, or, as we say, all adaptations, are capable of interpretation in terms of the process of selection, we must refer this great array of the most diverse protective devices to natural selection; and again, as among animals, we receive the impression that the organism is, to a certain extent, really capable of producing every variation necessary to its maintenance. Literally speaking, this would not be correct, but at any rate the number of adaptations possible to each form of life must be an enormous one, so great, indeed, that ultimately every species does secure protection for itself in some manner and in some degree, whether it be by the production of a poison or a nauseous substance within itself, or by surrounding itself with thorns or spines. And if it be, in a certain sense, a matter of 'chance' whether a plant has taken to one method of defence or to another, according as its innate constitution favoured the production of one rather than of any other, yet it would not be easy to prove, even in the case of the purely chemical means of protection, that these would have occurred in the same distribution and concentration as a necessary result of the metabolism of the plant, even if they had not been useful and consequently augmented by selection. But in the case of the mechanical means of protection this mode of explanation fails as utterly as that of the direct effect of the conditions of life. Why the holly should have spinose leaves beneath and smooth ones above can never be deduced from the constitution of the species.

While the protective adaptations of plants against the larger herbivores always point to natural selection, our appreciation of the adaptability of plants, and at the same time of the potency of natural selection, will be strengthened still more if we turn our attention for a little to the arrangements which prevent the extermination of plants by the lower and small animals.

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.

Among chemical means of protection must be named the pure bitter stuffs, such as are found in the species of gentian, the milkwort (Polygala amara), and in many other plants, and also the curious 'oil-bodies' of the liverworts.

But some plants also defend themselves against the attacks of snails by mechanical means.