THE VERNAL MIGRATION 19 



to the quality) of the food-supply ; for, though a district 

 may be adapted to, and inhabited by, a certain number 

 of such birds, yet, if an enormous additional influx of 

 foreigners be suddenly thrown upon it, its resources may 

 then become unequal to the increased demand, and a 

 proportionate exodus, or redistribution, must follow. 



Such, and cognate cases, however, are merely incidental 

 factors, and not the first cause of migration. There are, 

 moreover, many cases in which no such factors appear to 

 operate. 



Many theories in explanation of the migratory instinct 

 have been advanced. Some are, at any rate, ingenious ; 

 but, unless they rest on some solid basis, partake more of 

 a poetic than of a scientific character. The erection of 

 imaginative hypotheses, in support of which it is easy to 

 collect a mass of what looks like circumstantial evidence, 

 but which are incapable of direct proof, is of dubious 

 utility. Causes, no doubt, can be assigned to every effect, 

 a reason to every fact ; but it is perhaps wiser, with our 

 finite knowledge, to admit that there yet remain things 

 which cannot be explained. 



At the risk of appearing to neglect in practice what I 

 have just preached, I will venture briefly to refer to one 

 theory — which appears to stand on somewhat more 

 tangible foundations. This is the theory of the Polar 

 origin of life, which was first suggested in relation to the 

 origin of plant-life at the North Pole by Professor Heer 

 and Count Saporta. That the deductions of these 

 philosophic minds applied with equal force to the genesis 

 of bird-life was, in the first instance (I believe), suggested 

 by Colonel H. W. Feilden, C. B., in 1879. The subject 

 has more recently been treated by my old friend, the 

 late Canon Tristram, in relation to its bearings both on 



