46 THE RIDDLE OF MIGRATION 



with an aquatic animal of comparable attainments 

 while the latter, in its relatively dense medium, is 

 easily outdistanced by travellers in the air. A 

 winged insect may cover distances that the most 

 highly organised terrestrial vertebrate could not 

 contemplate. We may therefore reasonably expect 

 to find among birds the greatest and most spectacular 

 of migrations and our expectations are duly fulfilled. 

 But the ability to travel is not synonymous with 

 migration which is a particular type of travel with 

 quite distinctive features. We can formulate a 

 definition that clearly delimits the term. None 

 better than Gadow's exists and this we propose to 

 adopt. Migration is "the wandering of living crea- 

 tures into another, usually distant, locality in order 

 to breed there; this implies a return, and the double 

 phenomenon is annual. All other changes of the 

 abode are either sporadic, epidemic or fluctuating 

 within lesser limits. "^ This can be applied to all 

 animal migrations, even to those of fish which breed 

 but once in a lifetime. When they have reached a 

 certain age, the adults may travel far up rivers, or 

 across the ocean to breed and then die. The young, 

 however, come down the rivers in their youth to 



1 H. F. Gadow, Migration, in zoology. Encyclopaedia Bri- 

 tannica, 11th ed., vol. 18, p. 433-437. 



