MIGRATION OF BIRDS. 141 
newly inhabited localities might gradually produce a hardier 
race; winter forbade them staying in their new home all 
the year, but their absence from their first home had made 
it possible for the home food-supply of winter to be greater. 
Thus, both what we might call the mother-country birds 
and the colonials might be able to live together for the short 
period of winter. That habit of expansion once begun— 
and northern regions, or in turn southern, becoming warmer 
as the glacial periods passed away—might give rise, very 
likely did, to the habit of migration. It might be when the 
colonials came home, they, because they were hardier, and 
in a reduced home-supply of food, might exterminate the 
home birds, so that in time, after the habit of expansion had 
become inborn, the whole family of the species would become 
colonials and yet return to their ancestral home, though 
having now no wish to stay there all the year round. But 
this is anticipating. 
The method of enquiry into the cause of migration, 
I believe, was along another line. Enquiry began where 
the imagination was most arrested; for example, the 
‘stork and the swift know their appointed time, the 
swallows are back to their familiar haunts all over the 
country at once, and from all parts of the copses are heard 
the pleasing melody of the willow-warbler. It was therefore 
the insect-feeding birds, or what we actually know now to be 
the long-distance migrants, that set men a-thinking on the 
mystery of what we now call migration. Hence we had, 
to begin with, the Hibernation Theory—-a theory that has 
given no little fascination to the pages of Gilbert White of 
Selborne. Let me quote from his immortal book: “A 
‘clergyman of an inquisitive turn assures me that when he 
was a great boy, some workmen, in pulling down the battle- 
ments of a church tower early in the spring, found two or 
three swifts among the rubbish, which were, at first appear- 
ance, dead, but on being carried towards the fire, revived. He 
told me that out of his great care to preserve them, he put 
them in a paper bag and hung them by the kitchen fire, 
where they were suffocated.” Of course that clergyman was 
telling a lie. Again: “As a gentleman and myself were 
walking on the 4th of November 1771, round the sea-banks at 
