tion, reared their broods, clearly, if all things needed for their comfort were to be obtained, it cannot be supposed that these same birds would unnecessarily retrace their long flight to the distant South. This suggests that if we are correct in assuming that birds first appeared in a tropical climate, and from such climate migration started, it is probable that by gradually prolonging their northern visits and accustoming themselves to northern insect and vegetable life, these regions became populated by their resident species. It is evident that the present migratory species are simply compelled to return, and three compelling causes are demonstrable. Primarily, the sudden increase of cold at the close of the brief northern summer, which starts southward those farthest at the north. This accession of intense cold necessarily decreases the amount of food, and the birds are now forced to find it elsewhere. Farther and farther south they come, just in advance of the cold, and slower and slower they proceed, as they enter our more temperate latitude, and here, resting as it were, they linger until a keen frost kills their insect-food, and, scattering the leaves, robs them of their main shelter from their enemies, happily fewer now than formerly; and now still southward they proceed, until they reach a home in lands blessed with perpetual summer.
We have now traced these migratory species from south to north, and back to their southern habitat, and endeavored to point out the several operating causes of the movement as we did so. We have already suggested the possibility of migration being an inherited habit not now necessary. Now, be this true or not, it is evident that the habit is not so fixed a one that ordinary changes in surrounding conditions do not greatly influence it. This, we think, is shown by the irregularity of the movement that really occurs, and the tendency on the part of many species to modify the habit by occasionally halting much to the south of their usual breeding-grounds, and by remaining later and later in autumn; and, again, by the fact that many birds are now only partially migratory, and others by occasionally migrating simply in search of food, thus exhibiting, as it were, traces of a habit they have long lost, as to its full meaning and accomplishment.
In the migration of a bird, then, we see simply a temporary sojourn in a distant locality for the purpose of rearing its offspring in safety; the cause being implied by the term "safety," i. e., freedom from enemies, and an abundance of food.
