196 The Outline of Science 



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 larvse 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. 



