Friends and Foes 259 



undue multiplication. And one reason why plants intro- 

 duced into other lands sometimes run riot there, and even 

 extirpate the natives, is just this, that they are foreigners, 

 and that there is at first no animal to keep them within 

 bounds. If there had been some common bird to eat the 

 seeds of the wild artichoke or "thistle," for instance, 

 when first it reached South America, it could not have 

 gained the upper hand so entirely as it has done. At 

 present the plant itself is eaten to some extent by horses 

 and mules, but only when other forage is scarce, as its 

 spiny leaves are a great protection, and make it actually 

 formidable to most quadrupeds. Goats might manage it, 

 but otherwise it seems that birds are what are wanted to 

 keep it in check. Eventually, however, man may prove 

 to be its "appropriate enemy," and will cut and stack it, 

 as he has begun to do with its cousin, the Scotch thistle, 

 in Victoria, which proves excellent winter food for cows, 

 when thus treated, as the spines lose their stiffness when 

 dried. 



Foreign plants are, however, sometimes at a disadvan- 

 tage when introduced into a new country, as they may 

 meet with enemies unknown in their native land, and 

 against which, therefore, they are undefended. But 

 generally speaking, foreigners are much let alone at first, 

 for most animals are suspicious of anything new and 

 unfamiliar; and nearly all, especially of the mammalia, 

 far surpass us in keenness of scent. They "live in a 

 world of odors," most of them with their noses near the 

 ground, always on the qui vive to notice anything strange; 

 and generally speaking, what is new and strange that they 

 mistrust and avoid. 



When they have attained a fair size most plants can 



