ANIMALS INTRODUCED UNAWARES 419 



petition for food and room, which leads to the weakening 

 of one or other of the competitors. But this may be said, 

 that since the accidental introductions owe their presence 

 to concealment, they are as a rule smaller in size and less 

 conspicuous than man's imported favourites; and also, that 

 the former are on the whole more troublesome and pesti- 

 lential, for while the deliberate introductions do damage 

 mainly because they have betrayed the character man had 

 attributed to them, there can be no question about the char- 

 acter of the creatures which have escaped man's vigilance 

 their chief end seems to be to work mischief. In pro- 

 portion to their numbers the alien stowaways which become 

 established in a country include more economic pests than 

 the native fauna they invade. But after all this is exactly 

 what is to be expected: for a native association has been 

 brought by long trial and struggle to the dead-level of old 

 age Nature's balance of power; while the newcomers, freed 

 from the enemies which had developed along with them in 

 their former home and had kept them in check, and finding 

 in the country of their adoption no similar restraint, increase 

 and do damage to their heart's content. So the evil continues 

 until the very numbers of the pest induce some member of 

 the invaded fauna to turn upon them as a convenient food 

 supply, or until man devises some cunning means for their 

 destruction, or counters the foreign interloper by deliberately 

 introducing its foreign enemy (see Counterpests, p. 259). 



In the following pages are indicated the modes and re- 

 sults of the unforeseen introduction of animals to Scotland. 



272 



