126 THE EVOLUTION THEORY 



scanty, and the food available for the grazing animals is, at least at 

 times, very scarce ; on this account, if the plants are to survive there 

 at all, they must be armed with the most perfect means of protection 

 possible against the attacks of hungry and thirsty animals. The 

 struggle for existence in relation to such enemies is much more severe 

 than in more luxuriant regions, and the protection by thorns has 

 been developed to the highest possible pitch of perfection; species 

 which were unable to develop this protection died out altogether. 

 Hence the cactuses of Mexico, and the many thorny bushes and 

 shrubs of the hot, and, in the summer, dried-up stony coast-lands of 

 the Mediterranean in Spain, Corsica, Africa, and other countries. 

 This so-called ' Prigana scrub ' embraces a number of species, whose 

 nearest relatives in our climate are not provided with spines, as, for 

 instance, Genista hispanica, Onobryckis comuta, Sonclius cervicovnus, 

 Euphorbia spinosa, Stachys spinosa, and others. 



Why do so few thorny plants grow on the rich and well-watered 

 Alpine pastures 1 Probably because there is to be found there a rich 

 and luxuriant plant-growth which can never be wholly exterminated 

 by the grazing of animals, so that an individual species would not, 

 by developing thorns, have gained any advantage in the way of 

 increased capacity for existence. 



But these Alpine grazing grounds serve well to illustrate how 

 great may be the advantage which protective devices give to a species. 

 Much to the annoyance of the herdsmen, who endeavour to extirpate 

 them as far as possible, enormous masses of rhododendrons often 

 cover whole stretches, because their hard silicious leaves cannot be 

 eaten, and many other plants despised of cattle flourish and increase 

 on the grazing runs, like the repulsively bitter, large Gentiana 

 asclepiadea, the malodorous Aposeris fast Ida, and various ferns of 

 disagreeable taste. 



The advantage derived by plants from the possession of any 

 kind of protective device against grazing animals is perhaps best of 

 all seen in the ' shrubbery,' which on every Alp is to be found in the 

 immediate neighbourhood of the herdsman's hut. There, where the 

 cattle daily assemble, and where the soil is continually being richly 

 manured by them, we always find a large, luxuriantly growing 

 company of the poisonous aconite, the bitter goosefoot (Chenopodium 

 bonus henricus), the stinging-nettle, the thistle (Cirsium sjiino- 

 sissimum), the ill -smelling Atviplex, and some other inedible species, 

 while the palatable herbs are gradually exterminated by the cattle 

 which daily gather round the hut (Kerner). 



To sum up. We have seen that there is among plants an 



