834 LIFE : OUTLINES OF GENERAL BIOLOGY 



but there was a lowering of temperature toward the end of the 

 period, which may well have been fatal. Moreover, there is evidence 

 that the low-lying coastal lands underwent natural drainage, so that 

 the old amphibious haunts disappeared. Over-specialisation probably 

 counted for a good deal, but it looks as if climatic changes had been 

 potent factors in the disappearance, as in the emergence, of the 

 Dinosaurs — in many ways the most remarkable group of animals 

 that the earth has ever seen. 



It is not our object here to discuss the Dinosaurs themselves, but 

 several prominent facts must be noted: they spread over the earth, 

 and essayed both air and sea; on a very moderate estimate they 

 laisted from 5 to 10 millions of years ; they showed a diversity of struc- 

 ture surpassing that of mammals. Some were herbivores that ground 

 up coarse vegetation, and others, without teeth, sucked up juicy 

 marsh plants. Some were small carnivores preying on other little 

 creatures, including eventually the minute and timid mammals 

 that were beginning to show face, while others of gigantic size were 

 "the most terrible devourers of flesh the world has ever seen". As to 

 size, the smallest was the size of a cat; the largest had a length of 

 80 feet, and a probable weight of 40 tons. They entirely disappeared 

 at the close of the Cretaceous; but they did not whoUy die, for they 

 had meanwhile given origin to both birds and mammals. But while 

 these are scions of two Dinosaur stocks, they are not to be regarded 

 as direct descendants; and the large fact remains that none of the 

 highly-successful groups of Dinosaurs survived the end of the 

 Mesozoic. All were "lost races". 



In regard to the vegetation in the days of the Dinosaurs, i.e. in 

 the Mesozoic (Triassic, Jurassic, Cretaceous) Era, it was very different 

 from that of to-day. Some of the Palaeozoic plants, such as horsetails 

 and club-mosses and old-fashioned ferns, still persisted, occasionally 

 with vigour, but the characteristic feature was the abundance of 

 G5nnnosperms, that is to say, seed-plants with naked seeds. The 

 dominant groups were Conifers, Cycads, and Maiden-hair Trees. 

 Most characteristic were the Cycads, and "it is impressive to find 

 that a class now insignificant in extent then dominated the world". 

 One great authority on the evolution of plants has calculated that 

 out of all the Mesozoic land-plants in the days of the Dinosaurs, one 

 in every three was a Cycad; and another authority computes that 

 two out of every five land plants of this era were Cycads. But Cycads 

 are unfamiliar plants to-day. 



It must be noticed, however, that in the Upper Cretaceous, when 

 the Dinosaurs were beginning to show hints of racial senility, there 

 was an apparently sudden emergence of the Flowering Plants or 

 Angiosperms that are familiar to us to-day. The Cycads dwindled 

 and the Flowering Plants took their place ; the Dinosaurs disappeared 

 and the warm-blooded Birds and Mammals began to make headway. 



