THE MYSTER Y OF MIGRATION 203 



the * constant endeavour to regain and re-people the 

 area once occupied during pre-glacial times ' is a 

 conscious effort on the part of birds, though his 

 language almost suggests it. This he calls the 

 4 impulse to migration/ though why he should not 

 call it the ' instinct of migration/ is not very clear. 

 The 'propensity' in all the young birds must be 

 prior to actual experience, and independent of 

 instruction in many cases, such as that of the young 

 cuckoo, in which the parent bird leaves the young 

 to find their way back to Central Africa from 

 Kew Gardens or Richmond Park. 



Mr Dixon seems in difficulties between the 

 theory that the ' impulse ' to migrate is a trans- 

 mitted instinct due to descent from ancestors who 

 once lived in the northern regions, and a ' corollary ' 

 from his Law of Dispersal, that birds never retreat 

 before adverse conditions, but, ' if overtaken, perish, 

 so far as the species is exposed to them.' If the 

 members of a species, say, of the swallow tribe, 

 which were exposed to the glacial period, died, and 

 did not migrate, and only those were left which 

 were secure in ' refuge areas ' (parts which the ice 

 did not reach) survived, these Jattter could have 

 had no recollections of their old home to suggest 

 migration to their descendants. This ' law ' that 

 species all tend to migrate northwards in spring to 



