THE INSURGENCE OF LIFE 171 



is up to the time of its leaving its summer home, and how 

 rarely it is seen after that time is past.' 



Still more remarkable is the fact of spatial regularity. 

 For in a few cases (doubtless to be increased) we have 

 conclusive proof of a bird's return to its birthplace. 

 A swallow marked as a youngster with an aluminimn 

 ring has been known to return the following year, not 

 merely to the same county or parish, but to the same farm- 

 yard — a striking instance of precision in the sense of 

 locahty, and of a constitutional home-sickness bringing 

 the bird back from its winter- quarters (probably in Africa) 

 to its birthplace in England. The same return to the 

 original homestead has been proved in the case of the 

 house-martin and the stork, and is certainly one of the 

 most wonderful facts about migration. 



Concrete Problems of Migration. — One of the im- 

 portant questions which patient investigations, hke those 

 of Mr. Eagle Clarke, are in process of answering, concerns 

 the routes which birds follow in their migratory flight. 

 On the basis of observations made at lighthouses and 

 lightships and at strategic inland stations, it has been 

 possible to map out certain favourite routes. Equally 

 useful results have rewarded ' the ringing method ' pursued 

 by Dr. Thienemann at Rossitten, Dr. Mortensen in Holland, 

 Aberdeen University and the editor of British Birds in 

 Britain, and by others on the Continent and in the United 

 States. 



The locahties where any particular kind of bird was 

 originally ringed and was subsequently captured, are 

 registered on a map (a different one for each kind of bird), 

 and as the records of rings acciunulate in the course of years 

 the distribution of dots or crosses on the map begins to 



