258 Habit and Instinct. 



been seen crossing the image of the moon in astronomical 

 telescopes, under conditions from which it is inferred that 

 they were flying at an elevation of from one to three 

 miles. In dark and cloudy nights their cries may be 

 heard by any one who has ears to hear; and it is said 

 that in some species special cries are developed at this 

 time only. 



Such are the elementary and familiar facts with regard 

 to migration. The essential problems are : — Is the habit 

 truly instinctive, even in its details ? How did it originate ? 

 What is the nature of the impulse prompting the bird to 

 migrate ? What are the conditions under which it is 

 called forth ? And how do the birds find their way ? 



If not instinctive, the habit must be attributed to 

 tradition in the sense before defined. On this view the 

 migratory host is always led by older birds who have them- 

 selves been led by their predecessors, and so on ever since 

 the habit originated. This seems to be the hypothesis 

 adopted by Prof. Palmen, who ascribes the due per- 

 formance of the migration flight to experience. On this 

 view the congenital factor is a vague and indefinite innate 

 impulse recurrent at certain seasons. The course taken 

 is seemingly a matter of " follow my leader." But if, as 

 Temminck stated, and as Herr Gatke, with his fifty years' 

 practical experience in Heligoland, is convinced from a 

 vast mass of accumulated observations, the young birds 

 in a great number of species invariably -preceiie the old by 

 some weeks (the reverse being the case with cuckoos), the 

 hypothesis of tradition is, for these species, seemingly out 

 of court, and the mystery of migration deepens. "That 

 a chiffchaff, whose daily occupation for months has been 

 to pick grubs from the trees, and who has never left his 

 favourite wood, should suddenly, some evening, be seized 

 with an uncontrollable impulse to start for North Africa, 



