CAUSE OF MIGRATION 163 



this peculiar current of air ? and who of such fortunate 

 persons knows that the motion of their wings under such 

 conditions is ' involuntary like that of the heart " ? 

 Finally, what is the cause of the " paroxysm " ? for, with- 

 out knowing that, to attempt to explain the observed 

 facts of migration is an attempt to explain obscurum per 

 obscurius. 



When a satisfactory answer is given to these questions, 

 it will be time to inquire whether this " latest accepted 

 theory " of migration sets the matter in any clearer light, 

 or whether it is not as arrant nonsense as was ever foisted 

 upon an innocent public, even at the height of the " silly 

 season." The last paragraph of the writer's letter, I may 

 remark, has nothing in it of consequence. Granting that 

 the migratory impulse is instinctive, it is, like other in- 

 stinctive practices, followed as far as circumstances will 

 allow.* 



Then follows an admirable statement of the original 

 causes of migration, and of the modes of migration, 

 ending with the question : 



" How is it that birds find their way back to their 

 old home ? ' This seems to me the most inexplicable 

 part of the whole matter. I cannot even offer an 

 approach to its solution. . . . Here I have no theory 

 to advance, no prejudice to sustain. I should be thank- 

 ful indeed for any hypothesis that would be in accord- 

 ance with observed facts. . . . The solution is probably 

 simple in the extreme possibly before our eyes at this 

 moment if we could but see it but whosoever discovers 

 it will assuredly deserve to have his name remembered 

 among those of the greatest discoverers of this or any 

 age. 



With the caution perhaps even excessive that 

 was so characteristic of him, Newton would never permit 

 himself to advance any general theory of migration, nor 

 was he even satisfied with any of the theories suggested 



* Nature, September 24, 1874. 



