114 WILD TRAITS IN TAME AXOIALS. 



no exception, for, as every one knows, it has a 

 dark mark down its shoulder. Now, why should 

 the other stripes which once existed, and which 

 are still seen upon the zebras and occasionally 

 upon the horse, have disappeared in the case of 

 the donkey ? or rather, why should this particular 

 stripe which characterises him have remained 

 after all the others have faded ? According to 

 the first principles of Darwinism — which are now 

 fairly familiar to everybody — this peculiarity must 

 have been at some time of vital use to its possessor 

 or nature would not have preserved it. The only 

 use that I can think of for the dark stripe down 

 a donkey's shoulder is something to do with his 

 mountain habitat ; for although his grey coat 

 might fairly assimilate with the colours of the 

 rocks, his very bulk might reveal him to enemies, 

 unless the even colouringf was somewhat broken 

 up. We find that nearly all animals which are 

 protectively coloured exhibit certain lines and 

 other markings which seem to be adapted for 

 this end. The most notable exceptions are the 

 white and tawny creatures whose habitat is either 

 among the monotonous snow-fields or the sandy 

 deserts. As a rule, nature does not present 

 laro-e surfaces of one colour, but almost invari- 



