Evolution Going On 195 



is one of the methods of evolution to fill every niche of oppor- 

 tunity. 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 Atlan- 

 tic. 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 ac- 

 quiesce, 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 



