CHAP. IV Shifts for a Living 6i 



very numerous — one is tempted to ask whether there may 

 not be in many cases some explanation apart from the action 

 of natural selection upon casual changes. May not the 

 similar surroundings and habits of mimickers and mimicked 

 have sometimes something to do with their resemblance ; 

 may it not be that the presence of the mimicked has had a 

 direct, but of course very subtle, influence on the mimickers ; 

 is it altogether absurd to suppose that there may be an 

 element of consciousness in the resemblance between oriole 

 and friar-bird ? 



I o. " Masking " is one of the most interesting ways 

 in which animals strengthen their hold on life. It is best 

 illustrated on the sea-shore, where there is no little struggle 

 for existence and much opportunity for device. There many 

 animals, such as crabs, are covered by adventitious dis- 

 guises, so that their real nature is masked. Elsewhere, 

 however, the same may be seen ; the cases of the caddis- 

 worms — made of sand particles, small stones, minute shells, 

 or pieces of bark — serve at once for protection and conceal- 

 ment ; the cocoons of various caterpillars are often masked 

 by extrinsic fragments. The nests of birds are often well 

 disguised with moss and lichen. 



But among marine animals masking is more frequent, 

 "Certain sea-urchins," Mr Poulton says, " cover themselves 

 so completely with pebbles, bits of rock and shell, that one 

 can see nothing but a little heap of stones ; and many marine 

 molluscs have the same habits, accumulating sand upon the 

 surface of the shell, or allowing a dense growth of Algas to 

 cover them." 



This masking is in many cases quite involuntary. Thus 

 the freshwater snails {Lymnceus) may be so thickly covered 

 with Algae that they can hardly move, and some marine 

 forms are unable to favour or prevent the growth of other 

 organisms upon their shells. But how far this is from be- 

 ing the whole story is well known to all who are acquainted 

 with our shore crabs. For though they also may be invol- 

 untarily masked, there is ample evidence that they some- 

 times disguise themselves. 



The hermit-crabs are to some extent masked within 



