Notes on Migration in General 291 



Holly berries are also abundant, but as they are yet quite 

 green (September 22), the fact is not apparent to many. 

 There were but few acorns last year, there are many now, 

 and on both varieties of the oak. Finally, all our sycamores 

 are now laden with seeds. 



The causes determining the extraordinary prevalence of 

 seeds on certain trees in certain years are probably meteoro- 

 logical ones. 



NOTES ON MIGRATION IN GENERAL. 



It may be taken for granted that hunger is the primary 

 cause of the roaming of all animals ; it explains the regular 

 migration of certain birds between countries far separated, as 

 well as the localised migrations of the Alpine hare and the 

 North American deer. Dr. Robert Brown observes that 

 "just as in hibernation there is every gradation between 

 ordinary sleep and the long-continued dormancy so designated, 

 so it is possible to trace numerous steps connecting the 

 ordinary roaming about of an animal in search of food with 

 the persistent flight or march in one definite direction at a 

 date so determinate that it may be reckoned upon to within 

 a few days." 



Mammals are naturally less migratory than birds, their 

 movements being hindered by the ocean. But the same law 

 governs the movements of those that display the migratory 

 instinct, they move northwards in spring and south in 

 autumn. The reindeer and Arctic fox are the most pro- 

 nounced migrants amongst mammals. Of irregular migrants, 

 the Norwegian lemming is the best known. Mr. Duppa 

 Crotch, who made a careful study of its migrations, considered 

 that the southward movements of this mammal are stimulated 

 by an instinct to reach land long since submerged in the 

 Atlantic. He writes (alluding to swallows) : " It appears 



