130 THE EVOLUTION THEORY 



Wo may say, indeeil. that almost all wild Phanerogams are 

 protected in some degree against snails, and this almost suggests the 

 ([uestion : What then is left for the snails to feed on if everything is 

 thus arnuMl against them? But, in the first place, there remain our 

 cultivated plants, which, like the garden lettuce (Laduca), arc quite 

 without defence ; and secondly, the snails often eat the plants only 

 after thi'V have been rooted up and lie rotting on the ground, that 

 is, when the protective ingredient has been dissolved out by the rain ; 

 finally, no means of protection, as I have often said already, is abso- 

 lute or effective against all snails. Many of these are, as Stahl calls 

 them, ' specialists.' Thus, the large slug of our woods eats the 

 poisonous fungi which are rejected by other snails, and in the same 

 way there are many other specialists which, however, are not likely 

 to eliminate unaided the plants to which they have adapted them- 

 selves. There are certainly also omnivorous forms, like the field-slug 

 (Liniax agrestis), to which we have referred so often, and Ariuii 

 empiricoruin, the red slug, but just because these eat so many kinds 

 of plant they are less dangerous to any one species. 



These manifold devices for protecting plants against the depre- 

 dations of snails afford another proof that innumerable details in the 

 organization of plants, as of animals, must be referred to natural 

 selection, since they are capable of interpretation in no other w^ay. If 

 these protective devices were to be found only in isolated plants, we 

 might perhaps talk of ' chance ' ; we might refer them to the inborn 

 constitution of the plant, which made the production of bristles, or 

 bitter stuffs, or the deposition of silicic acid a necessity, and wdiich 

 ' happened ' to make the plants distasteful to certain snails. But as it 

 appears that all plants are protected against snails, one in this way, 

 another in that, this objection cannot be sustained. Furthermore, 

 some of the beautiful experiments made by Stahl to prove the pro- 

 tective effect of these devices showed, at the same time, that they 

 were not in themselves indispensable to the existence of the plant ; 

 maize, for instance, develops a plant perfectly capable of life, even 

 though silicic acid be withheld, and the acid is, therefore, not an 

 clement essential to its constitution, but a means of protection against 

 voracious animals. The clearest proof of this is afforded by plants 

 like the lettuce (Laduca), which formed protective stuffs in the wild 

 state, but have lost them altogether under cultivation, through disuse, 

 as we shall see more precisely later on. As the eyes of animals which 

 live in darkness have degenerated, so the plants which have been 

 taken under the protection of man have lost their natural means 

 of defence, because these were no longer necessary to the maintenance 



