140 Biology in America 



ica, Africa, India and Australia known as Antarctica and 

 Gondwana Land which is supposed to account for the simi- 

 larity of many of the forms of life in the two regions, not 

 only of birds and mammals but of reptiles, amphibia, fishes, 

 invertebrates and plants as well. 



While the early mammals disappeared for the most part 

 without issue there were among them some which were elected 

 to serve as progenitors of the mighty tribe which has since 

 peopled the earth. Whence came the present monotremes 

 and marsupials is matter of much doubt, their relationship 

 to the primitive members of these groups being uncertain, 

 but the origin of modern carnivores is pretty certainly to be 

 found in the ancient creodonts. 



After the close of the Cretaceous period with the rise of 

 the western part of the North American continent and con- 

 sequent draining of the great inland sea, which formerly 

 stretched from the Arctic Ocean to the Gulf of Mexico, there 

 appeared extensive swamps or fresh water lakes in what is 

 now North Dakota, Montana and the Rocky Mountain and 

 Great Basin regions and British Columbia. These various 

 lakes did not all appear at once, but succeeded one another 

 with succeeding changes in elevation and form of the land. 

 It was now that the lignite beds of North Dakota and Mon- 

 tana were laid down, covering an area approximately 60,000 

 square miles in extent. 



"The climate, as shown by the plants, was much milder 

 and more uniform than that of the Recent epoch, though 

 some indication of climatic zones may already be noted. The 

 vegetation was essentially modern in character; nearly all 

 our modern types of forest-trees, such as willows, poplars, 

 sycamores, oaks, elms, maples, walnuts and many others, were 

 abundantly represented in the vast forests which would seem 

 to have covered nearly the entire continent from ocean to 

 ocean and extended north into Alaska and Greenland, where 

 no such vegetation is possible under present conditions. Nu- 

 merous conifers were mingled with the deciduous trees, but 

 we do not find exclusively coniferous forests. Palms, though 

 not extending into Greenland, flourished magnificently far to 

 the north of their present range. On the other hand, the 

 Paleocene flora of England points to a merely temperate cli- 

 mate, while that of the succeeding Eocene was subtropical. ' ' 4 



Upon the land and in the lakes were laid down deposits 

 of wind-driven dust or loess and volcanic ash or tufa, while 

 the streams deposited in their deltas sand and gravel carried 

 down from the higher lands in which they took their rise. 

 The majestic Rocky Mountains of today were then in their 



4 Scott, locus citatus, pp. 102-103. 



