264 Master Minds of Modern Science 



sea, proof, too, that sea mammals can change their way 

 of life. 



Another instance which Sir Arthur gives of a sea 

 animal invading dry land is that of the robber crab of 

 Christmas Island. Christmas Island is in the Indian 

 Ocean, two hundred miles south of Java, and is famous 

 for its great beds of phosphates, which are very valuable 

 as a fertilizer. It was the Challenger Expedition which 

 made this discovery, and the whole cost of that expedi- 

 tion was repaid from royalties deriving from the sale of 

 these phosphates. 



The robber crab, a fairly big creature, is plentiful on 

 Christmas Island ; it has gained its name because, like the 

 American trade rat, it is fond of getting into houses 

 or workshops and stealing things. One has been seen 

 making off with an empty meat tin and using this as a 

 protection for its tail. It also climbs coconut trees and 

 breaks off the nuts. Then it climbs down, tears off the 

 husk of the nut, breaks a hole in the shell with its immense 

 claw, and spoons out the sweet milk with one of its legs. 



A queer beast indeed, and, as Sir Arthur says, specially 

 interesting in the story of evolution, because beyond doubt 

 it was once a marine animal. And it betrays its origin by 

 the fact that once a year it goes back to the seashore to 

 lay its eggs. The eggs are dropped in the sea, and the 

 young crabs, when hatched, live and swim in the salt 

 water for some considerable time. Then they come back 

 and creep on the shore, and at last become strong enough 

 to go inland and live there. 



Sea creatures breathe by means of gills; these are 

 feathery growths, and inside them the blood runs through 

 numerous small veins and takes oxygen from the water 

 which bathes the gills. Land animals breathe by means 

 of lungs which are inside the body. How then can a 

 gilled creature live on land? If you examine a robber 

 crab you find that it still has traces of gills, but on the 



