2o EVOLUTION OF LIFE 1.9- 



million years each in America, the Mississippian and Pennsylvanian. 

 Throughout this long time conditions varied widely in different parts 

 of the world. In the early Mississippian there were many swamps in 

 North America. In the northern hemisphere the Pennsylvanian was 

 probably a time of warm, moist conditions, with no cold winters, 

 but there are signs that for part of this time India and Africa were 

 covered with an ice-sheet. The coal measures show us the remains of 

 the forests of spore- and seed-bearing plants that were then pro- 

 duced, and the land conditions evidently favoured the life of the 

 Amphibia. 



The Permian probably constitutes a single 45-million-year period, 

 with very active orogenesis, leading to a more arid climate, perhaps 

 showing large seasonal changes, with deserts in some parts of the 

 world and glaciation in others. These conditions continued into the 

 Triassic, when the continents lay high. The reptiles, first found in 

 the Permian, developed throughout the Triassic and flourished in the 

 succeeding Jurassic period, which probably lasted 45 million years. 

 The Cretaceous period, during which the thick chalk deposits were 

 laid down, probably lasted for rather more than 60 million years, 

 including two major cycles of inundation. The lower Cretaceous 

 certainly included extensive periods of flooding, when there were 

 large shallow seas. Then later, towards the end of the upper Creta- 

 ceous, there were extensive orogenic movements, the Laramide 

 revolution, producing the Rockies and the Andes. The temperature 

 was warm until near the end of the Cretaceous, and we do not know 

 what condition led to the break that is found between the animals of 

 the Cretaceous and Eocene. Some groups of dinosaurian reptiles seem 

 to have died out suddenly, but it is important to notice that not all 

 disappeared at the same time, for instance, the stegosaurs and 

 pterodactyls (p. 569) disappeared well before the end of the Cretaceous. 

 However, it is probable that great changes went on at the end of this 

 period, and we may guess that a factor leading to the development of 

 the birds and mammals was the great rise of the continents, perhaps 

 accompanied by a fall in temperature over wide areas that had enjoyed 

 warmer weather. As always, when we look closely at such problems, 

 we are appalled by the vast lengths of time involved and the scanty 

 nature of our clues about them. The land lay very high at this time, 

 and the apparent abruptness of the break between Cretaceous and 

 Eocene fauna may be an artifact due to the scarcity of fossils. In 

 North America there is evidence from terrestrial deposits of a long 

 Paleocene period between the Cretaceous and Eocene. 



