18 DISPERSAL AND MIGRATION. [PART 1. 
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 
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