PROTECTIVE ACAPTATIONS IN PLANTS 127 



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 



