ture, 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.

The Case of the Robber Crab

On some of the coral islands of the Indian and Pacific Oceans there
lives a land-crab, Birgus, which has learned to breathe on land. It
breathes dry air by means of curious blood-containing tufts in the upper
part of its gill-cavity, and it has also rudimentary gills. It is often
about a foot long, and it has very heavy great claws, especially on the
left-hand side. With this great claw it hammers on the "eye-hole" of a
coconut, from which it has torn off the fibrous husk. It hammers until a
hole is made by which it can get at the pulp. Part of the shell is
sometimes used as a protection for the soft abdomen--for the
robber-crab, as it is called, is an offshoot from the hermit-crab stock.
Every year this quaint explorer, which may go far up the hills and climb
the coco-palms, has to go back to the sea to spawn. The young ones are
hatched in the same state as in our common shore-crab. That is to say,
they are free-swimming larvae which pass through an open-water period
before they settle down on the shore, and eventually creep up on to dry
land. Just as open-water turtles lay their eggs on sandy shores, going
back to their old terrestrial haunt, so the robber-crab, which has
almost conquered the dry land, has to return to the seashore to breed.
There is a peculiar interest in the association of the r