'TARTAR INVASIONS 219 



like that of the crazed lemmings and the insects that 

 rush away in myriads to inevitable destruction, is a 

 danger as well as an advantage to the bird. The 

 reasons so far given for these great "Tartar invasions 

 of Europe," as they were called, first in 1863 an( ^ on 

 two subsequent occasions, are just about as con- 

 vincing as any of the other migration theories I have 

 mentioned. This is, according to Professor Newton's 

 pronouncement on the subject, that the bird increases 

 beyond the capacity of the country it inhabits to 

 sustain it, for it is known to be prolific, breeding 

 twice and even three times a year, and is also well 

 able to escape from its enemies on account of its 

 wariness and swift flight; that when the overpopula- 

 tion reaches a certain point, the birds fly away in 

 search of fresh pastures. 



But what made them fly to Europe when all Asia 

 was open to them to choose from ? He might have 

 taken his idea from Kirby and Spence. A better 

 explanation is wanted, and it may be found in the 

 fact that the impulse to migrate is occasioned by an 

 extraneous force which is itself subject to violent 

 permutations, which have a disordering effect on 

 the sense. 



As to my private views on the cause of migration, 

 that is a matter of minor consequence since I am 

 better as an observer than a thinker. It has been 

 hinted at all through this chapter, and it has been 

 suggested twice before by two independent observers. 

 From the first I was led inevitably to the conclusion 



