_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 migrants 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 multiplication 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 {19}immediately connected with the process of reproduction. Some, as the salmon, enter rivers; others, as the herring and mackerel, approach the coast in the breeding season; but the exact course of their migrations is unknown, and owing to our complete ignorance of the area each species occupies in the ocean, and the absence of such barriers and of such physical diversities as occur on the land, they are of far less interest and less connected with our present study than the movements of birds, to which we shall now confine ourselves.