206 Proceedings of the Royal Physical Society. 



bird population will always, however energetic we are, remain so small that 

 any shooting of birds for the sake of chance ringed examples would be 

 ridiculous. It is also worthy of note, that a very large proportion of 

 marked birds are reported as found dead, found injured, or as captured 

 and released. 



Turning now to the all-important question of the value of such results as 

 may be obtained by bird-marking, we find that the method differs from other 

 methods in that it approaches the problems from the individual aspect — it 

 begins with individual birds, working from them to general movements. A 

 marking record implies that there are two or more moments in the life of a 

 particular bird when one is able to state with certainty its whereabouts and 

 various other facts. When a large number of such records have been collected 

 and correlated, we have an array of general facts which could not possibly be 

 ascertained by other means. 



But to estimate the value of such facts we must for a moment consider 

 the nature of the problems before us, if I may recall a few well-known 

 points. Perhaps the greatest and most difficult problem of migration is that 

 of its origin — its ultimate cause. To a considerable extent, we are sure of 

 the purposes served by migration, its raison d'etre : and we are perhaps well 

 on the road to an understanding of the immediate factors which stimulate 

 the migrational habit into being with the recurrence of the seasons ; but the 

 question of the origin of the habit still lies completely within the realms 

 of conflicting theory. A matter of theory and hypothesis it must doubtless 

 ever remain ; but we may at least put our theories to the test of facts, and 

 eliminate those that are found wanting. And, in the meantime, we must 

 seek the facts. 



One thing is obvious, and that is that migration is a far too complex and 

 also a far too regular phenomenon to be created anew each season, merely 

 under stress of circumstances ; moreover, we know that migration begins 

 before the need is in the least pressing. The more or less indefinite wander- 

 ings of some sea-fowl, or the irregular dispersals of some other birds, may be 

 attributed to immediate causes, but a deeper seated origin there obviously 

 must be for the highly developed migrational habit of some of our more 

 typical travellers. 



A little consideration will show how our theorising regarding this origin 

 is rendered futile for lack of a certain kind of facts. For instance, we have 

 the perhaps rather far - fetched theory that the migrational habit was 

 established by some great meteorological change in the far past — say, for 

 instance, by a Glacial Epoch (as has been suggested) which drove the birds 

 resident in northern latitudes towards the Equator, and made them form 



