200 BRITISH BIRDS. 
from Persia, through Turkestan and Cashmere, along the mountains 
of South-west Siberia as far east as Krasnoyarsk and Yenesaisk ; and 
in the latter locality appears to have driven out the White Wagtails and 
to have divided them into two colonies. As I ascended the Yenesay 
from the Arctic circle the White Wagtail abounded on the banks of the 
river until I neared Yenesaisk about lat. 59°, when suddenly it disappeared, 
and its place was taken by M. personata; but about halfway between 
Yenesaisk and Lake Baikal, Middendorff found the White Wagtail again, 
and Taczanowsky describes it as common near Lake Baikal and in West 
Dauria. The isolation of this colony appears to have been facilitated by 
the fact that in autumn the birds migrate eastward instead of westward to 
winter in West China, Burma, and India. Some ornithologists have 
described birds from this colony as distinct under the name of M. duk- 
hunensis; but I am unable to detect any difference between examples from 
Lake Baikal, North Siberia, India, or Spain. North-west European 
examples have somewhat narrower white margins to the wing-coverts. 
The British ornithologist who leaves his native island and crosses over 
to the continent may ramble over any farm on the shores of the German 
ocean without finding much difference between the birds of the two 
countries. Most of them he will recognize as old acquaintances ; a few 
which he looked upon as great rarities im England he will find to be 
common on the other side of the water; and some which at the first glance 
he may suppose to be common British birds, he will find on closer exami- 
nation to differ slightly from their insular representatives. To the latter 
eroup belongs the White Wagtail. On the continent from Calais to 
Hamburg this bird everywhere takes the place of the Pied Wagtail. The 
two birds have the same habits, nearly the same song and call-notes, make 
a similar nest, and lay similar eggs. There seems to be no difference be- 
tween them whatever, except in the colour of the back and the sides of the 
neck. The Pied Wagtail is obviously an island form of the White Wag- 
tail, and has probably been differentiated since the passing away of the 
Glacial epoch. During the warm climate which appears to have succeeded 
this cold period in these latitudes, the White Wagtails of the British 
Islands were probably ‘isolated from their continental brethren, and 
possibly having fewer enemies (both birds and beasts of prey being com- 
paratively less abundant on islands than on continents) sexual selection 
was not prevented by protective selection from providing them with a 
special bridal dress at the spring moult. In those days the isolation of the 
two areas of distribution was probably complete ; and we may fairly assume 
that the two species only began to invade each other’s territory 1m com- 
paratively recent times, since the winters have become sufficiently cold to 
compel them to be more or less migratory in their habits. Hence we find 
that although the White Wagtail is so rare in our islands, it is not, like 
