GREAT STEPS IN ORGANIC EVOLUTION 841 



Middle Ages) is often dated at about 60 million years ago, and the 

 beginning of its third and last period, the Cretaceous, at about 

 30 million years ago. The Cretaceous Period gets its name from the 

 abundance of chalk deposits, but there are many other kinds of rocks. 

 It lasted for some 15 million years, and ended in great crust-move- 

 ments, sometimes called the "Laramide Revolution". 



In the Lower Cretaceous there was a continuation of the charac- 

 teristic Mesozoic plants, such as the Cycads and their relatives, 

 but the flowering plants were beginning to come to their own, and 

 one of the great events of the Cretaceous Period was the growing 

 dominance of Monocotyls (like reeds and palms) and Dicotyls (like 

 magnoHas and laurels). Expressing it technically, we may say 

 that Angiosperms began to gain on Gymnosperms. The flora of the 

 Lower Cretaceous is ancient, that of the Upper Cretaceous is 

 modern. 



In Cretaceous times the evolution of reptiles continued apace, 

 and numerous specialised types arose. But even in the Lower 

 Cretaceous there were indications of the waning of the small- 

 brained giants. There were new Flying Dragons or Pterodactyls, 

 some of which probably caught fishes from the surface of the sea. 

 One of these strange types. Marsh's Pteranodon, had a wing span 

 of over 24 feet and an extraordinary skull about 6 feet long, but 

 it probably did little in the way of active flight. From the restricted 

 dimensions of the pelvic girdle it has been inferred that the newly 

 hatched young were very small. Not related to the Pterodactyls 

 were the true birds, which included in Cretaceous times some modern 

 types. Of great interest was the increase of smaU and primitive 

 mammals which were gradually to supplant their reptilian pre- 

 decessors. The overthrow of the reptilian dynasty was associated with 

 crustal changes, culminating in the Laramide Revolution, and one 

 of the factors was probably the obliteration of the low-lying coastal 

 haunts. Towards the end of the Cretaceous there seems also to have 

 been a widespread lowering of temperature. In any case it was a 

 dramatic time when small furry quick-witted mammals began to 

 supplant the large, brawny, well-armoured, but slow-going reptiles. 



Tertiary or Cenozoic. — For our purpose in this book it is 

 unnecessary to follow the sub-divisions of the Tertiary or Cenozoic 

 Era, which may well be called the Age of Mammals, when brains 

 began to be of much greater survival value than brawn. But it is 

 interesting to notice that the first period, the Eocene, was marked 

 by numerous archaic or generalised mammals, which were replaced 

 in the Oligocene and Miocene by the modern orders, such as Carni- 

 vores, Ungulates, Bats, and Monkeys. A general land-elevation led 

 to the widespread dominance of certain types of vegetation, such as 

 grass-lands, which do not require a high degree of moisture. The 

 clothing of large areas of the earth with a garment of grass must 



