Page:Popular Science Monthly Volume 7.djvu/204

192 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 differing 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 for the habit—one inducing it in the spring, another compelling the migrating bird 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 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 need fly from coming rigors of scorching 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 movements. The progenitor of our score of warblers, the one tyrant flycatcher, from which all our species have sprung, the vireos, the goatsuckers, 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 portion of the northern world, and at the close or closing of that wonderful 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 originating 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 origin and proper meaning of which are wholly 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