Friends and Foes 303 



Probably each plant has its own appropriate enemy 

 ■ — really a friend to the race — whose office it is to check 

 its undue multiplication. And one reason why plants 

 introduced 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 other- 

 wise 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 stiff- 

 ness when dried. 



Foreign plants are, however, sometimes at a dis- 

 advantage when introduced into a new country, as 

 they may meet with enemies unknown in their native 

 land, and against which, therefore, they are un- 

 defended. But generally speaking, foreigners are 

 much let alone at first, for most animals are sus- 

 picious 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 odours,' 

 most of them with their noses near the ground, always 

 on the qui vive to notice anything strange ; and gener- 



