120 THE EVOLUTION THEOKY 



superabundant nourishment until the plants on which they depended 

 were themselves consumed. 



When we inquire into the means whereby plants evade such 

 a fate we are astonished at the endless diversity of the devices 

 employed. 



Let us consider first of all the menace to plants from the larger 

 herbivores, from elephants and cattle down to the hare and the roe- 

 deer ; we find that many plants are protected by poisons, which 

 develop in the sap of their steins, leaves, roots, and fruits. The juicy 

 and beautifully leaved Belladonna (Atropa belladonna) is never 

 touched by roe-deer, stags, or other herbivores, and the same is true of 

 the thorn-apple (Datura stramonium), the henbane (Hyoseyanw.s 

 niger), the spotted hemlock (Conium maculatum), the danewort of 

 our woods (Sambucus ebulus), and many others; they all contain a 

 poison. Like the unpalatable butterflies, these unpalatable plants are 

 also furnished with a warning sign of their undesirability, namely, 

 a disagreeable odour, perceptible even by man, which scares off animals 

 from touching them. The development of this through natural 

 selection presents no very serious difficulty. 



But, strangely enough, there are not a few poisonous plants in 

 which Ave, at least, are unable to detect any such warning sign. 

 Among these are the blue aconite (Aconitum), the black hellebore 

 (Hellehorus niger), the meadow-saffron (Colchicum autumnal 'e), species 

 of Gentian, of spurge (Euphorbia), and others. Yet these are avoided 

 by deer, roe-deer, chamois, hares, and marmots, and our cattle, horses, 

 and sheep also usually leave them untouched. A case has, however, 

 been reported from the valley of the Aur, on the lower Khine, which 

 seems to contradict this. On the rocky grass-slopes of the valley the 

 poisonous hellebore (Hellehorus viridis) grows in great abundance, and 

 the sheep of that region, which were wont to graze on the slopes, 

 avoided these plants. But some sheep from another part were im- 

 ported into the valley, and these ate the hellebore, with the result that 

 many died. If these poisonous plants, then, were furnished with a 

 warning sign such as a disagreeable odour, not perceptible to us, we 

 should have to assume that the imported sheep had a less acute sense 

 of smell than the others, which is not impossible in domesticated 

 animals. If there were no such warning sign, then it must have been 

 not an instinct but a continuous tradition which prevented the native 

 sheep from touching the inedible plants. 



A more naive interpretation of nature than that of our day 

 would have regarded the fragrant ethereal oils developed in the seeds 

 of many plants, as in those of fennel, cummin, and other Umbelliferous 



