MIND AND M&MOlir OF BIRDS. 23 



tion and of summer heat was followed by the 

 gradual coming on of what is called the 

 glacial age, when vast accumulations of ice, in 

 the form of glaciers, swept down from the far 

 north and destroyed all life in America, as far 

 south at least as the Ohio River valley. Dur- 

 ing the time this enormous body of ice was 

 accumulating and moving down in the form 

 of a glacier, toward the gulf, our birds began 

 to feel a desire to move away southward be- 

 fore the chilly invader. This desire was not 

 born in a day, or a year, or a century; it 

 slowly grew by hereditary descent and accre- 

 tion, so to say, operating differently in differ- 

 ent species. Some birds by infinitesimal de- 

 grees modified their physiques to conform 

 somewhat to the exigencies of the climatic 

 changes; others, following the call of natural 

 desire, crept away in the direction of warm 

 sea-currents and genial sunshine until they 

 were huddled in some lost Atlantis, some 

 tropical garden of preservation washed by 

 tepid ocean-streams over which the glacial 

 rigor could not prevail. Then came another 

 oscillation of Nature. The tropical region 

 began to return toward the pole, drawing the 

 birds along with it, and now here they are 

 again swarming in to the land out of which 

 the ice-king drove them hundreds of centuries 

 ago! 



As I swung in my hammock under the 

 grimy beams of my gin-house, listening to 

 the mocking-birds' songs and to the mellow 

 moan of the sea, I began to analyze and com- 

 pare all the foregoing facts, and it seemed to 

 me that I discovered the solution of this mys- 

 tery of bird migration which has troubled 

 naturalists so long. 



During the countless centuries of the qua- 



