as the name implies, includes all of North America, Europe, Asia north
of India, and the Himalaya mountains, northern Africa where the great
Sahara forms the natural boundary, and all islands belonging to the
north temperate and north frigid zones. Many have divided this great
belt into Palearctic and Nearctic, but the intermingling of species
between northeast Siberia and Alaska seems to make such a distinction
impracticable. But these distinctions should be and are retained in the
divisions of the Holarctic. When we understand that at least one-third
of the species found in the Nearctic are also found in the Palearctic,
we shall understand why these two are grouped under one region. There
are no orders, and there seem to be no families which are found in the
Holarctic and nowhere else. Indeed, it is difficult to find even genera
which do not have some species ranging into the Neotropical, Ethiopian
or Indian. But among the species we find many. Indeed, there are few
species which nest in both the Holarctic and in the regions bounding it
on the south, and many of these are found only on the southern
boundaries of the Holarctic. In our part of the Holarctic, that is, the
Nearctic, the familiar birds about us do not nest also in the tropical
regions.