128 THE EVOLUTION THEORY 



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 (Hijypuris). 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 (Oxcdis 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 vobertiamim). 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 b} r a thread. The 

 mints (Mentha) and the dittany (Dietamnus 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. 



First there are the various kinds of bristle arrangements, which 



