18 DISPERSAL AND MIGRATION. [part r. 



best suited to a bird's habits, are probably the causes which mark 

 out the exact limits of the range of each species ; to which must 

 be added the prevalence of enemies of either the parent birds, 

 the eggs, or the young. In the Malay Archipelago pigeons abound 

 most where monkeys do not occur ; and in South America the 

 same birds are comparatively scarce in the forest plains where 

 monkeys are very abundant, while they are plentiful on the open 

 plains and campos, and on the mountain plateaux, where these 

 nest-hunting quadrupeds are rarely found. Some birds are 

 confined to swamps, others to mountains ; some can only live on 

 rocky streams, others on deserts or grassy plains. 



The Phenomena of Migration. — The term " migration " is often 

 applied to the periodical or irregular movements of all animals ; 

 but it may be questioned whether there are any regular mi- 

 grants but birds and fishes. The annual or periodical movements 

 of mammalia are of a different class. Monkeys ascend the 

 Himalayas in summer to a height of 10,000 to 12,000 feet, and 

 descend again in winter. Wolves everywhere descend from the 

 mountains to the lowlands in severe weather. In dry seasons 

 great herds of antelopes move southwards towards the Cape of 

 Good Hope. The well-known lemmings, in severe winters, at 

 long intervals, move down from the mountains of Scandinavia in 

 immense numbers, crossing lakes and rivers, eating their way 

 through haystacks, and surmounting every obstacle till they 

 reach the sea, whence very few return. The alpine hare, the 

 arctic fox, and many other animals, exhibit similar phenomena 

 on a smaller scale ; and generally it may be said, that whenever 

 a favourable succession of seasons has led to a great multipli- 

 cation of any species, it must on the pressure of hunger seek 

 food in fresh localities. For such movements as these we have 

 no special term. The summer and winter movements best 

 correspond to true migration, but they are always on a small 

 scale, and of limited extent; the other movements are rather 

 temporary incursions than true migrations. 



The annual movements of many fishes are more strictly 

 analogous to the migration of birds, since they take place 

 in large bodies and often to considerable distances, and are 



