xvi INTRODUCTION. 
been, in comparatively recent times, subject to but little change. On the 
other hand, the genera in the two arctic regions are crowded with imper- 
fectly segregated species, which require for their explanation comparatively 
recent and important changes in the climate. These are to be looked for 
in the gradual approach of another glacial epoch. In the warm period 
which permitted the growth of pines almost at the north pole the whole of 
Europe, North Asia, and North America enjoyed a semitropical climate, 
and the variation of species throughout the arctic regions was probably 
very small. As the climate in Lapland and Siberia gradually changed, the 
birds living there gradually changed also ; and thus we find now in many 
Palearctic species a semitropical form in West Europe which is connected 
by a series of intermediate forms with an arctic form in Siberia, which 
again gradually changes in the east until in China the West-European 
form is reproduced. In some cases the series is completed by tropical 
species which have evidently been driven south by the glacial period and 
have never returned. I have endeavoured to interest the reader wherever 
possible in some of these near relations of our British birds, many of them 
so closely allied as to be only subspecifically distinct. 
The number of birds included in the British list is about 380. Of these 
126 are residents, though many of them have only slender claims to be 
considered so. Some of them are principally known as winter visitants, 
a few only remaining to breed, chiefly in the north of Scotland ; others 
really belong to the summer visitants, but a sufficient number remain 
during the winter to entitle the species to be considered a resident one. 
Fifty-five species regularly visit our islands every summer for the purpose 
of breeding ; but many of these are becoming rarer every year, partly in 
consequence of the persecution to which they are subjected on their 
arrival, and partly from the destruction of their breeding-grounds by the 
drainage of marshes and the reclamation of waste land. Forty-one species 
may be regarded as winter visitants; but many of these wander still 
further south during midwinter, being principally seen on our shores in 
spring and autumn. The birds contained in these categories form a total 
of 222 species which are fairly entitled to be considered British birds. 
It has, however, been the practice of ornithologists to consider any bird 
British which has even once been obtained in our islands in a wild state. 
Of the accidental visitors which thus reach our shores many have occurred 
only once; but others have been met with much oftener, though some of 
the recorded occurrences must be accepted with considerable hesitation. 
Birds often escape from confinement; a mistake is made in the identifica- 
tion of the species; and in very few cases are we able to trace clearly the 
pedigree of individual examples so as to leave no reasonable doubt of their 
authenticity, for skins are very frequently changed or transposed either by 
accident or design. The number of these accidental visitants to the 
ncn 
