12 HISTORY OF THE EUROPEAN FAUNA. 



modes of progression. It may walk or fly. In both, 

 however, there is a third mode of transport an in- 

 voluntary one. The deer may be suddenly seized by 

 a flood whilst crossing a river, and carried far away 

 without necessarily coming to grief. The beetle in 

 a similar manner could be transported to a distant 

 country, or it might be caught in a whirlwind and 

 blown hundreds of miles off. 



We may thus distinguish between the natural or 

 active and the accidental or passive means of distribu- 

 tion of animals. The active mode of dispersal again 

 may be only migratory, as in most animals, or periodic 

 and migratory, as in some birds and fishes. It is of 

 course the tendency of every species to spread in all 

 directions from its original home, provided it does not 

 encounter obstacles, such as want of food, unsuita- 

 bility of climate or soil, or barriers such as mountains, 

 rivers, or the sea. Birds might be thought to be little 

 interfered with by any of these barriers, but, as Dr. 

 Wallace has shown, they are almost as much affected 

 by them in their distribution as mammals are. 



This then is the ordinary migratory distribution. 

 Periodic distribution obtains with migratory birds 

 and fishes. The annual flight of swallows to their 

 northern summer residence comes under the heading 

 of periodic migration or distribution, but apart from 

 this, the swallow must seek to extend its range by 

 the ordinary method, like every other animal. 

 Similarly, the herring migrates periodically into 

 shallow water to spawn, only to return again to its 



