THE MIGRATION OF INLAND BIRDS. 119 



either terminus of the journey. A warbler that winters 

 in Florida and breeds near the Arctic circle is influenced 

 by a cause that exists at each terminus, or rather by 

 two differing causes, each peculiar to the location, for it 

 is wholly incredible that it is the same cause that induces 

 both the visit to northern regions and the return to a 

 southern clime; therefore there must be at least two 

 reasons for the habit one inducing the bird to migrate 

 northward in the spring, another compelling it to return 

 in the autumn. If it be possible now to demonstrate 

 what these causes are, and how the same cause can influ- 

 ence all migratory birds, considering that their habits are 

 otherwise so totally different, it will not then necessarily 

 follow that it was the originating cause of the habit. 

 "When, indeed, did this migration commence ? How far 

 back into the world's geological history must we go to 

 trace the first bird that was forced to seek another and 

 far-distant land wherein to rear its young and find for 

 its offspring and itself sufficient food ? "What conditions 

 of heat and cold, land and water, summer and winter, 

 then obtained, that birds must needs fly from coming 

 rigors of scorching sun, or ice and floods, or perish where 

 they w r ere? Was it from living in such a world that 

 migration originated, and became, strangely enough, 

 characteristic of only a fraction of the whole number? 

 How, too, could birds have learned the oncoming of dis- 

 astrous times, and know just where to seek a safe harbor 

 and secure rest ? Clearly it could have been only by a 

 very gradual accumulation of experiences extending over 

 many generations, before the few progenitors of our many 

 birds gained the happy knowledge that here in the north 

 we have months of sunny summer weather and a wealth 

 of pleasant places. I shall not go back, then, of the Gla- 

 cial period, but rest content with it as having been the 



