All through the millions of years during which animals have tenanted the
earth and the waters under the earth, there has been a search for new
kingdoms to conquer, for new corners in which to make a home. And this
still goes on. _It has been and is one of the methods of evolution to
fill every niche of opportunity._ There is a spider that lives inside a
pitcher-plant, catching some of the inquisitive insects which slip down
the treacherous internal surface of the trap. There is another that
makes its home in crevices among the rocks on the shore of the
Mediterranean, or even in empty tubular shells, keeping the water out,
more or less successfully, by spinning threads of silk across the
entrance to its retreat. The beautiful brine-shrimp, _Artemia salina_,
that used to occur in British salterns has found a home in the dense
waters of the Great Salt Lake of Utah. Several kinds of earthworms have
been found up trees, and there is a fish, Arges, that climbs on the
stones of steep mountain torrents of the Andes. The intrepid explorers
of the _Scotia_ voyage found quite a number of Arctic terns spending our
winter within the summer of the Antarctic Circle--which means girdling
the globe from pole to pole; and every now and then there are incursions
of rare birds, like Pallas's Sand-grouse, into Britain, just as if they
were prospecting in search of a promised land. Twice or thrice the
distinctively North American Killdeer Plover has been found in Britain,
having somehow or other got across the Atlantic. We miss part of the
meaning of evolution if we do not catch this note of insurgence and
adventure, which some animal or other never ceases to sound, though many
establish themselves in a security not easily disturbed, and though a
small minority give up the struggle against the stream and are content
to acquiesce, as parasites or rottenness eaters, in a drifting life of
ease.
More important than very peculiar cases is the broad fact that over and
over again in different groups of animals there have been attempts to
master different kinds of haunts--such as the underground world, the
trees, the freshwaters, and the air. There are burrowing amphibians,
burrowing reptiles, burrowing birds, and burrowing mammals; there are
tree-toads, tree-snakes, tree-lizards, tree-kangaroos, tree-sloths,
tree-shrews, tree-mice, tree-porcupines, and so on; enough of a list to
show, without mentioning birds, how many different kinds of animals
have entered upon an arboreal apprenticeship--an apprenticeship often
with far-reaching consequences. What the freeing of the hand from being
an organ of terrestrial support has meant in the evolution of monkeys is
a question that gives a spur to our imagination.