124 THE ESSEX NATURALIST.
toward the south? "In the Third Book of the Iliad, attributed
to Homer, who probably lived in the 12th century B.C., the
Trojan hosts are described as appearing "like the Cranes which
flee from the coming winter and sudden rain and fly with
clamour towards the streams of the ocean.'' In the fourth century
B.C. the prophet Jeremiah (viri., 7) refers to some of the spring
arrivals, "Yea, the Stork in the heavens knoweth her appointed
times; and the Turtle, and the Crane and the Swallow observe
the time of their coming." These were merely passing allusions,
but as early as B.C. 384-322 the migrations of birds were dis-
cussed by Aristotle. To give a brief description which would
serve as a guide to migration is not easy, as the subject is so full
of contradictions. For example, birds frequently do not take
the most direct route to their summer and winter quarters; in
fact, at some stages of their flight they may be flying from their
objective. The outline, which I now present, however, will be
of service in spite of its great limitations. There are said to be
about 13,000 species of birds, but many, perhaps a majority,
are not migratory. The globe might be divided very roughly
into three zones: the first, the Arctic and north temperate belt;
the second, the tropical belt; and the third the Antarctic and
south temperate belt. The birds of the first and third zones are
mainly migratory, those of the second mainly sedentary. The
birds of the first division chiefly move northwards to and south-
wards from their breeding quarters, and those of the third division
southwards to and northwards from their nesting localities.
Thus, after the completion of the nesting operations, the birds of
both these belts move towards the Equator and sometimes
beyond it. Some breeding species of the northern area, such as
the Knot, which are circumpolar in their summer quarters, have
a vast winter distribution in the Southern Hemisphere, including
the Cape, India, Australia, etc. Although the birds of the
southern area do not migrate to the same extent into the northern
hemisphere yet some of them, such as Wilson's Petrel and the
Great and Sooty Shearwaters, find their way very far to the
north, the first two sometimes reaching the fringe of the Arctic
Circle. With this skeleton of the world's bird-migration in our
minds we can better consider the movements of birds in relation
to the British Isles. In very high latitudes migration does not
present much difficulty, as practically all the birds are purely