MIGRATION. 175 



active survive to perpetuate the species. The 

 number of swallows often found dead on the 

 seashore washed up by the restless waves after 

 a storm affords a proof of this mortality. 



The regularity of these migratory movements 

 is wonderful. Fair weather or foul the puffin 

 turns up at his accustomed breeding - place 

 with clockwork regularity. The swift again 

 is peculiarly punctual in the date of his arrival. 



Mr Wallace seems most strongly of opinion 

 that food is the mainspring of migratory move- 

 ments. He says : — 



"It appears to me probable that here, as in so 

 many other cases, ' sin-vival of the fittest ' will be 

 found to have had a powerful influence. Let us 

 suppose that in any species of migratory birds, 

 breeding can as a rule be very safely accomplished 

 in a given area ; and further, that during a great 

 part of the rest of the year sufficient food cannot 

 be obtained in that area. It will follow that 

 those birds which do not leave the breeding area 

 at the proper season will suffer, and ultimately 

 become extinct; which will also be the fate of 

 those which do not leave the feeding area at the 

 proper time. Now, if we suppose that the two 

 areas were (for some remote ancestor of the 

 existing species) coincident, bub by geological 

 and climate changes gradually diverged from 

 each other, we can easily understand how the 

 habit of incipient and partial migration at the 

 proper seasons would at last become hereditary, 

 and so fixed as to be what we term an instinct. 

 It will probably be found that every gradation 

 still exists in various parts of the world, from a 



