It would at first seem from the above that the annual migratory movement would be in a direction due north and south, or south and north, according to season, and so in a general way it is; but there is no doubt that this simple movement is disturbed by many causes, chief among which is possibly the configuration of the land, which is found to give rise to considerable deviations, and that to an extent which is at present very imperfectly understood. It may be considered proved that the trend of a coast-line, the course of a great river, or the intervention of a chain of mountains, has a very appreciable effect on the direction taken by migrating Birds; but not one of these, nor all in combination, affords a sufficient explanation of all the deflexions, and will certainly not account for at least one remarkable fact, as it may now be regarded–the tendency of many Birds in Eastern Europe and part of Siberia to travel westward towards the close of summer or in autumn. This is shewn in several ways, but in none better than by the almost yearly occurrence in Britain at that season of examples of species which breed only in the Russian Empire. For, admitting that such examples are stray wanderers, which have lost their course, their appearance here is still useful in indicating the existence of the westward movement; and, with the evidence they furnish before us, we may judge whence come vast numbers of others–Starlings, Crows, Rooks, Jays, Larks, and what not, whose origin and starting-point it would be otherwise hard to trace or even surmise. Much has been written, especially in Europe, on so-called Lines of Flight, but as yet to little purpose, and indeed {19}scarcely any writers on the subject have had sufficient _data_ to form an hypothesis, so that it is not surprising that hardly any two agree in theory.[10] In other parts of the world there is still less ground for theorising, though in North America many valuable observations have been made; and these, in conjunction with those carried on in Europe, will no doubt in due time lead to satisfactory results as regards the Northern Hemisphere. Concerning the Southern our ignorance is almost complete.