THE CAUSE OF THE MIGRATORY MOVEMENT 145 



of migrants, we are naturally led to ask by what impulse such birds 

 are urged to set out on this journey to their homes. 



Nor again are dearth of food and a low temperature the motive 

 causes for migration in autumn. The higher purpose of this 

 mighty process is unmistakably to prevent the races of birds from 

 perishing from hunger and cold ; of this however the single indivi- 

 dual has no consciousness, nor in fact can it have any idea as to the 

 degree in which its home becomes less habitable with the advance 

 of the season, for all such species as are subject to regular migra- 

 tions generally leave their homes long before the scarcity of food or 

 the fall of temperature becomes such as to be no longer support- 

 able. Long before the approach of winter all these birds are in 

 latitudes, which, in mildness of climate and abundance of food, are 

 in no ways inferior to those which form their summer abodes, nor 

 do any of them return before spring has again rendered their homes 

 habitable ; in fact we may say that from the time they have 

 escaped from the egg they have lived in one continuous summer ; 

 hence none of them knows what winter, with all its discomforts, 

 means ; and accordingly none can be possessed of any innate ten- 

 dency for avoiding that, of the existence of which they have up to 

 the present remained entirely ignorant. Should a further particular 

 proof be required showing that neither lack of food nor cold is the 

 direct causes of the departure of birds on their migrations, I would 

 call attention to a fact which I myself have determined, viz. that 

 during the autumn migration, the young birds of the year leave 

 their homes from one to two months in advance of their parents. 

 Indeed, in the case of the Starling, the young birds which migrate 

 at the end of June are not followed by the parents until the end of 

 September. Now in this case scarcity of food can hardly have 

 been the cause for the departure of the young birds, since there 

 must have been a sufficient supply to enable the old birds to stay 

 three months longer. Nor can we lay down this earlier departure 

 to the influence of cold, for as a rule a rise rather than a fall 

 of temperature takes place in the months succeeding June. 



In a manner similar to that described in the case of the Starling, 

 the autumn migration of old and young birds proceeds almost un- 

 exceptionally among all species of migrants. 



According to Professor Newton's view, scarcity of food really is 

 the cause which compels the individuals of a species having their 

 homes in the extreme north to leave their breeding quarters for 

 districts farther south. There their arrival would lead to over- 

 crowding, which again would drive previous residents in these 

 regions to turn southwards, and so on, until all the birds have 

 arrived in latitudes providing them with an abundant food supply. 



K 



