MIGRATION 



the species may be met with at certain spots throughout the whole 

 .year, those wliich occur at one season are not always the same 

 individuals as those which occur at another — the particular 

 Thrush, Titmouse, or Finch, appearing in the winter not being 

 identical with that which appears in summer. Again, among 

 species of which some individuals are constantly present throughout 

 the year, a great accession to the numbers is made at the close 

 of the breeding-season by the influx of other individuals of the 

 same species bred in another district, though this influx generally 

 lasts for a comparatively short time, and the strangers pass on, 

 accompanied it may be, by some or even most of those that have 

 been reared on the spot in the season immediately preceding. 

 These species are the " Partial Migrants." 



It would at first seem from the above that the annual mi- 

 gratory 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 dis- 

 turbed by many causes, chief among which is possibly the 

 configuration of the land, which is found to give rise to con- 

 siderable 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 Eussian 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, Books, 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 



