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 larvæ 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 robber-crab with
the coco-palm, for that tree is not a native of these coral islands, but
has been introduced, perhaps from Mexico, by the Polynesian mariners
before the discovery of America by Columbus. So the learning to deal
with coconuts is a recent achievement, and we are face to face with a
very good example of evolution going on.
[Illustration: EARLY LIFE-HISTORY OF THE SALMON