y go from the centre of Norway to the east and west,
crossing valley, hill, and river in great masses. Many are destroyed by
birds and beasts of prey, but finally the survivors reach the Atlantic
on the Gulf of Bothnia and, for some strange unknown reason, plunge in
and die. Only enough remain from one season to another to propagate the
species. It is an immense co-operative suicide society.

Rivers and valleys are sometimes effectual barriers. On the plains of
the Amazon great numbers of animals are found on one side of the river
only; these have not been able to cross to the other. On the north side
of the Rio Negro are two varieties of monkeys, the _brachiurus conxion_
and the _jacchus bicolor_, which are unknown on the south side. Of
course, water-loving animals, such as seals, whales, and porpoises are
at home in the water and can swim for days without stopping. Quite a few
animals can swim for a short distance, but comparatively few for long
distances. In the early days in North America it was not uncommon for
buffalo to swim across the Mississippi River. Rats and squirrels often
migrate in great numbers. It oftentimes happens that Arctic animals
travel from one place to another on floating ice. In the South American
waters it is a common sight to see floating islands covered with plants
and trees upon which there are live animals; and while these animals are
likely to perish, they are oftentimes carried safely to land. Eagles
have often been instrumental in bringing new species of animals to
islands where they had previously been unknown, their purpose being to
provide food for their own young. Some of these animals would escape and
henceforth become citizens of their new habitation.

An interesting division of migrants is that of the casual travellers,
like the men and women who always remain at home except when special
business calls them away. Sudden climatic changes, or the scarcity of
food, often cause stay-at-home animals to make tours into new
territories. As a good instance, I might cite the case of three wolves,
which I saw entering Jackson Park in Chicago, during very severe weather
when Lake Michigan was frozen over. The morning papers stated that
because of forest fires in Michigan, and the extreme cold, which not
only made food scarce for the wild animals of Michigan, but froze the
Lake, many of them had come across the ice into the great Chicago parks
seeking food and shelter.

The subject of anim