CHAP. XVI.] _ 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 Ungulata. 



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 imited 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 



