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. 



