A MIGRANT IN MINIATURE 207 



as in the case of Swallows, which were supposed 

 to w,inter under water, and the case is worth con- 

 sidering. 



As to the reasons for migration, we have here 

 some chance, when we know more completely the 

 life-history of the birds, of finding them out ; and 

 at the outset, migration is in its essence simply the 

 periodical removal of an animal from conditions 

 which have ceased to be attractive to a locality 

 where it is, or expects to be, more comfortable. It 

 is not confined to birds, for some fish are well- 

 known migrants — one has only to cite the salmon — 

 and mammals also undertake migrations, as the 

 American bison used to do, and as sea-lions do still. 



A very interesting case is that of a small and 

 very local British land-snail {Helicodonta obvoluta), 

 now only found here on a limited area on the 

 South Downs, which executes a miniature migra- 

 tion from its hibernating haunt on the ground to 

 the branches of the beech-trees in spring, spending 

 the summer aloft, and in autumn coming down the 

 trunk to bury itself again. There is no essential 

 difference between the short journey of this humble 

 mollusc and the enormous transit of the world's 

 record migrant, the Arctic Tern, which breeds up 

 to the northern limit of land, and in our winter 

 reaches, on its southern migrations, even to the 

 cheerless shores of the Antarctic continent, there 

 to enjoy whatever sort of summer that unpromising 

 region can offer it. 



Migrations in elevation, made no doubt very 



