192 THE POPULAR SCIENCE MONTHLY. 



another, it is evident that the cause or causes of this movement is one 

 or more that operates at either terminus of the journey. A warbler 

 that winters in Florida, and breeds near the arctic circle, is operated 

 upon by a cause that exists at each terminus, or by two diifering causes, 

 each peculiar to its own location, and 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 causes 

 lor the habit one inducing it in the spring, another compelling the 

 migrating bii'd to return. If it be possible now to demonstrate what 

 these causes are, and. how the same cause can influence all migratory 

 birds, considering that their habits are otherwise so totally ditierent, 

 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 suflicient food ? 

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

 then obtained, that birds must need fly from coming rigors of scorch- 

 ing sun, or ice and floods, or perish where they were? 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 disastrous 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. 

 We will not go back, then, of the Glacial period, but rest content with 

 it as having been the starting-point in time of birds' migratory move- 

 ments. The progenitor of our score of warblers, the one tyrant fly- 

 catcher, from which all our species have sprung, the vireos, the goat- 

 suckers, and cuckoos, then very few in species, if indeed there were 

 more than one of each, must have been influenced by the presence of 

 the icy barriers that shut them off for the time being from a vast por- 

 tion of the northern world, and at the close or closing of that won- 

 derful period it may be that migration commenced, yet why and how, 

 we can but guess. Knowing that it commenced then or recommenced, 

 if previously a feature of bird-life, we have now to inquire what are 

 its apparent causes at present ; but, before inquiring into these, may 

 we not, after all, ask if migration be not an inherited habit, the origi- 

 nating causes of which are not now in operation? The conditions 

 not obtaining that necessitate migration, does it not become a case of 

 survival of habit, just as in man many customs now exist, the oi-igin 

 and proper meaning of which are M'hoUy lost? That this is true of 

 the migration of all birds we do not believe, but that it partially holds 

 good with some species we are fully convinced. As an inherited 



