4. Australian, embracing Australia, New Guinea, New Zealand and the southern Pacific.
5. Nearctic, roughly America, north of the Gulf of Mexico.
6. Neotropical, America south of the Gulf.
Newton suggested an alteration, a continuous northern region to he called the Holarctic Region, which embraces almost the whole of the Northern Hemisphere, and the division of the Australian into Australian and New Zealand Regions. Each of these southern regions is the winter home of some of the Holarctic birds, and it is a matter of dispute whether many of these originated in the northern or southern hemispheres. The value of these artificial divisions of the world is rather in the consideration of the conditions their varied climates and physical features present as attractions to birds in search of suitable nesting places and food supplies.
The study of Migration involves reference to the work of ornithologists of the past and present, the mass of contradictory literature already referred to, and we are repeatedly faced with the difficulty that some particular theory about the vexed questions of the cause or origin of migration, the height and speed at which birds travel, whether they do or do not follow routes, how they find their way, in what order they migrate, how and why
