158 The Geographical Distribution of Animals. — [March, 
flowed successive waves of life, as they each in turn became 
temporarily united with some part of the northern land. Aus- 
tralia appears to have had but one such union, perhaps during 
the middle or latter part of the Secondary epoch, when it re- 
ceived the ancestors of its Monotremata and marsupials, which it 
has since developed into a great variety of forms. The South 
African and South American lands, on the other hand, appear 
each to have had several successive unions and separations, allow- 
ing first of the influx of low forms only (Edentata, Insectivora, 
and lemurs), subsequently of rodents and small Carnivora, and 
latest of all of the higher types of Primates, Carnivora, and Un- 
gulata. 
During the whole of the Tertiary period, at least, the northern 
hemisphere appears to have been divided, as now, into an east- 
ern and a western continent, always approximating and some- 
times united towards the north, and then admitting of much in- 
terchange of their respective faunas, but on the whole keeping 
distinct, and each developing its own special family and generic 
types, of equally high grade, and generally belonging to the same 
orders. During the Eocene and Miocene periods, the distinction 
of the Palzarctic and Nearctic regions was better marked than 
it is now, as is shown by the floras no less than by the faunas of 
those epochs. Dr. Newberry, in his Report on the Cretaceous 
and Tertiary Floras of the Yellowstone and Missouri Rivers, 
states, that although the Miocene flora of Central North Amer- 
ica corresponds generally with that of the European Miocene, yet 
many of the tropical, and especially the Australian types, such 
as Nakea and Dryandra, are absent. Owing to the recent dis- 
covery of a rich Cretaceous flora in North America, probably of 
the same age as that of Aix-la-Chapelle in Europe, we are able 
to continue the comparison, and it appears that at this early pe- 
riod the difference was still more marked. The predominant 
feature of the European Cretaceous flora seems to have been the 
abundance of Proteacex, of which seven genera now living in 
Australia or the Cape of Good Hope have been recognized, be- 
sides others which are extinct. There are also several species of 
Pandanus, or screw-pine, now confined to the tropics of the east- 
ern hemisphere, and along with these oaks, pines, and other 
more temperate forms. The North American Cretaceous flora, 
although far richer than that of Europe, contains no Proteacee 
or Pandani, but immense numbers of forest trees of living 
and extinct genera. Among the former we have oaks, beeches, — 
