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 " invohmtary hke 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 iier 

 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. 



