74 LIFE : OUTLINES OF GENERAL BIOLOGY 



But there are also outside influences, which work hand in hand 

 with the internal impulses. The changes of the seasons are in some 

 measure the instigators of migration. Our summer visitors begin to 

 feel in autumn the increasing cold, scarcity, storms, and darkness. 

 In spring they may be provoked to return to the north by the 

 increasing heat and glare in their winter quarters. Yet we must not 

 think of these outside changes as compelling the birds to migrate; 

 they simply help to release an instinctive impulse or internal 

 prompting that has become enregistered in the race. This is a 

 difficult idea, and one of the reasons is that as man has very few 

 of these instinctive impulses, he does not find it easy to understand 

 creatures in which they are very powerful. 



In some cases migration is a social activity, for the migrants may 

 gather in great flocks and fly away together. So we must not exclude 

 the possibility that migrants of the same kith and kin excite one 

 another, and that some may be much more susceptible than others 

 to the internal and external trigger-pulling. Some yoimg black- 

 headed gulls which we hatched out and reared at college and after- 

 wards set free in a comfortable garden, where they were well sup- 

 plied with food, became interested at the time of migration in their 

 kindred who flow overhead, and they eventually joined them. 

 Matthew Arnold has a beautiful picture of a captive stork who sees 

 his fellows flying south : 



And as a stork which idle boys have trapped, 

 And tied him in a yard, in autumn sees 

 Flocks of his kind pass flying o'er his head 

 To warmer lands and coasts that keep the sun, 

 He strains to join their flight and from his shed 

 Follows them with a long complaining cry. 



How MiGK.\TioN Began. — Our fourth question is: How did migra- 

 tion begin? In all probability the evolution of bird-migration 

 is wrapped up with the history of climate. There were genial ages 

 long ago when our summer visitors were resident birds throughout 

 the north temperate countries of Europe. But slow changes of 

 climate towards cold made it diflficult for certain types to face the 

 severer winter. So they flitted for short distances southwards in 

 autumn, returning in spring to nest in cooler places with abundance 

 of water and food. As the change of climate became more and more 

 pronounced, the distance of the autumnal journey would have to 

 be increased; and it is reasonable to suppose that callous or fool- 

 hardy or stupid birds, who would not take a hint, were sifted out 

 of the race. A premium would he put on sensitive birds, on strong 

 fliers, on keen vision, on powers of picturing scenery and reviving 

 the picture. Thus the migratory custom might gradually evolve. 

 In thinking of the origins of migration, we must also remember 



