18 MIGRATION CHAP. 
the species may be met with at certain spots throughout the whole 
year, those which 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 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 
