CHAP. XVL] 
SUMMARY AND CONCLUSION. 
155 
Marsupials of America and Australia,- were all represented in 
Europe (and probably also in North America) during the earlier 
part of the Tertiary epoch. These facts, taken in their entirety, 
lead us to conclude that, during the whole of the Tertiary and 
perhaps during much of the Secondary periods, the great land 
masses of the earth were, as now, situated in the Northern 
Hemisphere ; and that here alone were developed the successive 
types of vertebrata from the lowest to the highest. In the 
Southern Hemisphere there appear to have, been three consider- 
able and very ancient land masses, varying in extent from time 
to time, but always keeping distinct from each other, and repre- 
sented, more or less completely, by Australia, South Africa, 
and South America of our time. Into these flowed successive 
waves of life, as they each in turn became temporarily united 
with some part of the northern land. Australia appears to have 
had but one such union, perhaps during the middle or latter part 
of the Secondary epoch, when it received 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 suc- 
cessive unions and separations, allowing first of the influx of low 
forms only (Edentata, Insectivora and Lemurs) ; subsequently of 
Eodents and small Carnivora, and, latest of all, of the higher 
types of Primates, Carnivora and TJngulata. 
During the whole of the Tertiary period, at least, the Northern 
Hemisphere appears to have been divided, as now, into an 
Eastern and a Western continent ; always approximating and 
sometimes united towards the north, and then admitting of much 
interchange 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 distinc- 
tion of the Palsearctic 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 Eeport on the 
Cretaceous and Tertiary floras of the Yellowstone and Missouri 
Eivers, states, that although the Miocene flora of Central North 
