INTRODUCTION. 
71 
said to be very extensive, possessing, with a few 
exceptions, the whole of this portion of the Fauna 
of northern and temperate Europe, those which 
are wanting being natives either of the extreme 
north or south, and just bounding on the forms 
which occur in the nearest extra-European divi- 
sions of the world, Asia or Africa. Many of the 
species are migratory, a consequence always ac- 
companying insular lands ; but the proportion is 
much less than in islands lying either more to the 
north or south, where the general climate is less 
temperate or uniform, and where the species from 
either extreme annually crowd, to perform the 
duties of incubation, or to spend the winter far 
from seasons, the rigour of which even their hardy 
nature could not withstand. And it is from the 
middle situation, as it were, of our islands that the 
birds more peculiarly southern or arctic are met 
with only as stragglers or occasional visitants, 
their instinctive wants being satisfied when they 
reach a climate which holds a middle class, and 
is subject neither to an arctic cold nor the intense 
heat of an almost tropical region. Another 
reason for the large proportion of species found 
in our Fauna, is the very great variation of country 
within its range. Sea-coasts of the most rugged 
and precipitous nature, or flat, sandy and stretch- 
ing into boundless salt-marshes and fens, afford a 
residence and breeding stations for many sea-fowl 
