MAY. 79 



aids them in spring, are able to live all the year in 

 a climate which exactly suits them, because it suits 

 the myriads of small things upon which they feed. 

 They escape both the grip of Arctic frost and the 

 blistering heat of the " hot weather " in the Tropics ; 

 for Englishmen instinctively refuse to call that season 

 of suffering by any name so suggestive of outdoor 

 happiness as "summer." But the original migrations 

 of birds were not, of course, nearly so extensive; 

 and the process no doubt commenced by the surplus 

 bird population of the year drifting with the wind in 

 search of food in autumn, and returning to their 

 homes, the wind again aiding them, in spring. Some 

 would be carried wide on the return journey, and 

 travel to distant regions, thus gradually extending 

 the northern range of their kind ; while in winter 

 those that travelled furthest southwards would fare 

 best, thus extending that range too. 



DELIBERATE TRAVELLERS. 



Thus regarded, the migration of birds becomes 

 a process of simple and easy growth, each kind dis- 

 covering, by the working of natural selection, the 

 distances and the kind of weather, that is, the approxi- 

 mate date, for travelling which suit it best. But to 

 some extent, of course, birds exercise free will in 

 their movements. Thus, rising higher and higher as 

 they fly to sea, they will refuse to pass out of sight 

 of one coast till they can see the opposite one. From 

 this results the habit of travelling birds to work up 

 or down a coast-line till they reach the narrowest 

 crossing. And in returning in spring they have the 



