WINGED VOYAGERS. 79 
we may readily conceive that, as the sun melted the 
ice at a greater distance in both directions from the 
equator, the ‘habitable area of the earth’s surface 
would gradually become enlarged. For the sake of 
vividness let us fancy ourselves living at that period 
of the world’s history. Let us select a point north 
of the equator where a given pair of birds can live 
in summer. ‘They find plenty of food there, and 
are comparatively undisturbed by other birds, and 
they therefore become attached to the place, most 
feathered folk having a strong “homing instinct.” 
When winter comes, they and their progeny are 
forced to retire to the south; but they do not for- 
get their pleasant summer haunt, their Mecca in 
the north, and therefore, at the approach of the 
following spring, they obey the home impulse and 
hie by easy stages to the beloved spot. Some of 
their number doubtless find it possible from time to 
time to push farther northward, and thus other 
breeding-haunts are selected. As the glacial ac- 
cumulations melt away, the whole, temperate region 
and a large part of the frigid zone become habitable. 
All this takes place by a very gradual process, re- 
quiring thousands of years, thus giving ample time 
for heredity to infuse the migratory habit into the 
nature of the birds. Every new generation would 
learn the route and other needful details from 
their predecessors, and thus the process would go 
on in an unending circuit year by year. 
After the foregoing was written, my attention was 
called to the following quotation from Dr. J. A. 
