THE CRETACEOUS PERIOD 431 



the Rocky Mountains. By the retreat of the inland sea at 

 about the same time, the continent was left more nearly in 

 its present condition than ever before. 



Climatic conditions. In the Mesozoic, as in earlier eras, 

 the climatic conditions left but scanty records from which 

 we may now draw inferences. Extensive red beds give evi- 

 dence of an arid climate over large areas in western United 

 States, Europe, and China; but such conditions may have 

 been due to the same local factors which produce deserts 

 to-day. It is thought that the growth of abundant corals 

 and other tropical animals in northern Europe in the Jurassic 

 period indicates a much warmer general climate than the 

 present. There are also differences in the faunas of northern, 

 middle, and southern Europe and North America which may 

 be due to climatic zones. That such zones have been in 

 existence throughout geologic history can hardly be doubted, 

 but, as already said, evidence of their presence in the earlier 

 periods is scanty. It is quite probable, moreover, that the 

 zones have been more distinct at certain times than at others. 



Evolution of higher types of animals and plants. When 

 the Mesozoic era began, the old Paleozoic ferns and seed 

 ferns were sinking into a subordinate place, as the conifers, 

 cycads, and other naked-seed plants came to the front. 

 Before the end of the era, however, even these were super- 

 seded in large measure by the modern flowering trees, shrubs, 

 and grasses. By this change the landscapes doubtless came 

 to look much more like those which we now see. 



Early in the Mesozoic era the reptiles were in the youth 

 of their race, rapidly developing and rising to their zenith 

 before the Comanchean period. Having mastered the life 

 of the dry lands, the shallow seas, the air, and even the open 

 oceans, they kept their -dominant place until the end of the 

 era. Their later years were marked by inability to com- 

 pete with the rising mammals, and it is perhaps for this 

 reason that they were soon relegated to the background. 

 Other groups of animals underwent corresponding, if per- 



