This cannot apparently be done for insects or foi birds; Newton accordingly unites the two into the Holarctic region.
Amongst broadleaved trees Juglans has a similar Holarctic range, descending to the West Indies; so has Aesculus, were it not lacking in Europe; it becomes tropical in South America and Malaya.
These facts have led some naturalists to include the Palaearctic and Nearctic regions in one, termed Holarctic, and to suggest transitional regions, such as the Sonoran, between North and South America, and the Mediterranean, between Europe and Africa, or to create sub-regions, such as Madagascar and New Zealand.
Faunistically, although not geographically, the Nearctic and Palaearctic areas must form the two subdivisions of one great unit, for which the " Holarctic region " is now the generally accepted term.
The HoLARCTIC Region, comprising North America and the extratropical mass of land of the Old World, may from an ornithological point of view be characterized by the Colymbi, Alcidae, Gallidae or Alectoropodous Galli, and the Oscines, which have here reached their highest development; while Ratitae, Tinami, Psittaci, and non-Oscine Passeres (with the exception of Tyrannidae extending into North America and Conurus carolinensis) are absent.