178 THE STORY OF THE BIRDS. 
around the north pole, on all the continents near it, 
at a time when this region was tropical in climate, as 
it evidently once was, judging from its fossils; and 
that as the great ice-cap formed over it and covered 
the earth, gradually extending away southward, per- 
haps to the Ohio Valley on our continent, it drove be- 
fore it many of the birds, fleeing for their safety. 
There was no opportunity to temper themselves to 
the climate, no chance whatever to live upon the bare 
ice only. But when summer came each year the great 
ice wave would recede a little, and the birds would 
follow it back and buiid their nests as near as possible 
to the old home location, placing them often, no doubt, 
under the very brow of the glacier. In time the ice 
receded to its present limit, and the habit once set up 
has caused many birds to follow it yet. Others have 
set the limit farther south, with all degrees of gra- 
dation, for the old tropical climate never came again 
to the arcties. 
If this view be correct it will be readily seen that 
the habit of migration thus set up and continued 
would prevent the traveling birds from becoming 
hardened to winter or adapting themselves to a win- 
ter diet, because the first cool blast, or even the dim- 
ming and lowering toward the south of the autumn 
sun, would send them south, doubtless, with the in- 
herited impression that the great ice billow was creep- 
ing down yet only a short distance north of them ; 
just as the little kitten while yet blind hisses at the 
odor of the friendly dog, because his tribe has so long 
been the enemy of its ancestry. 
