So on the whole, this region has not developed so rapidly as the Holarctic.
But these distinctions should be and are retained in the divisions of the Holarctic.
In the second place, the line of migration seems to have been outward from the Holarctic region.
In our part of the Holarctic, that is, the Nearctic, the familiar birds about us do not nest also in the tropical regions.
There are no orders, and there seem to be no families which are found in the Holarctic and nowhere else.