146 THE BIRDS OF HELIGOLAND 



This explanation would have much to commend it if all the indivi- 

 duals of a species left their breeding-places in the highest northern 

 latitudes simultaneously, and if all followed a north-to-south line of 

 migration. I have however shown myself, firstly, that the young 

 and old birds of a species migrate at widely different times ; and, 

 secondly, that a number of species perform their migrations on an 

 east-to-west line of flight. Moreover, if the movement were really 

 of the gradually progressive type indicated by the view expressed 

 above, we should expect to see it proceed at a steady measured 

 pace, whereas, on the other hand, we have it presented to us on 

 Heligoland in the form of an overpoweringly vast and rapid torrent 

 of migration, in which all the original force and simplicity of the 

 movements are preserved, and which dashes over the island with 

 the chaotic impetuosity of an irresistible cataract. 



Want of food cannot be considered as the cause of this boisterous 

 departure of the birds, for the simple reason that, even if such a 

 scarcity did arise in the immediate surroundings of their nesting- 

 places, they would endeavour to obtain provisions in districts some- 

 what farther removed, but certainly not by a sudden flight in a 

 straight line, extending over many hundreds, or even some 

 thousands of miles. Further, those species, the main business of 

 whose life proceeds in daylight, and which in darkness can do no 

 more than flutter about in aimless and unsteady fashion, would 

 certainly not defer their departure till sunset, in order to look for 

 food during the dark hours of the night. 



In recent times it has been the fashion to trace back the 

 migratory movements of animals, like any other inexplicable vital 

 activity, to the hereditary principle ; and the impulse for the won- 

 derful migratory flights performed by birds is said to have been 

 developed from isolated and accidental movements of an erratic 

 nature. Assuming this view to be correct, it nevertheless affords 

 no explanation as to the conditions which must of necessity have 

 accompanied a first excursion of this kind. Let us assume that a 

 bird, used to a uniformly warm climate and an abundance of food, 

 happened in spring, while in search of a mate or nesting-place, to 

 stray accidentally into latitudes so far north that the approach of 

 winter would plunge it into the direst necessity. Being as yet 

 ignorant of migration, our hapless straggler would, under such con- 

 ditions, rove about in a helpless fashion, half-starved, and numbed 

 with cold, and could only escape certain destruction if some lucky 

 chance should conduct it towards the south. Now in these circum- 

 stances it is surely more rational to assume that such a bird, taught 

 in the school of experience, would avoid for the future getting into 

 similar straits, rather than that it would voluntarily expose itself 



