138 ELEMENTS OF VERTEBRATE PHYLOGENY 



the immediate ancestors of the reptiles — which, with their dry, scaly 

 skins and a number of internal improvements, were the first vertebrates 

 to become quite divorced from the waters. 



The first land vertebrates must have had an easy time of it. Escaping 

 the fierce competition of the waters, they found themselves exploring a 

 new world in which they had no enemies. There was abundance of food, 

 for the plants had taken to the land eons before. The very ease with which 

 the land animals could spread and multiply encouraged the rapid pro- 

 duction of new types. And then, the inevitable happened — some of these 

 newer forms found the older ones good to eat. Competition on land 

 eventually became so keen that many reptiles, mammals, and even birds 

 returned to an aquatic existence. On land, their muscles had had to 

 sustain their weight and had become far more powerful than those of 

 fishes. Claws, beaks, and crushing teeth had also evolved, and with such 

 superior weapons many species found it easy to get a living in the water. 



The reptilian group flourished amazingly and ruled the world for tens 

 of millions of years through its aristocracy, the group we call the dino- 

 saurs. But even while the twenty-foot tyrannosauri were mangling the 

 ninety-foot diplodoci, the first of the mammals were furtively sneaking 

 about looking for dinosaur eggs to suck, and the first birds — derived 

 from tiny dinosaurs — were getting off the ground for short flights. The 

 reptiles which we have around us are a mixture of old and new. The 

 lizard-like Sphenodon (rapidly approaching extinction on a couple of 

 New Zealand islands) is the sole survivor of the rhynchocephalians, the 

 rest of which died with the dinosaurs. The turtles are of enormous 

 antiquity — turtles are among the oldest reptilian fossils we know of, 

 and they were already perfectly standard turtles "way back then'. The 

 ancestors of the crocodile group can also be traced back into the begin- 

 nings of the Age of Reptiles. 



The lizards, however, came into existence only recently as an offshoot 

 of the extinct mosasaurs. The snakes originated as legless lizards, so 

 very recently (as geological time intervals go) that the most primitive 

 of them, the boas and pythons, still have vestiges of the hind legs. Leg- 

 lessness has since arisen independently several times in different families 

 of existing lizards, but these snake-like forms are still true lizards. 



The mammals fall into three great divisions: the egg-laying mono- 

 tremes of which only the duck-billed platypus (Ornithorhynchus) and 

 the echidnas are left on earth; the marsupials, which originated in South 

 America and left primitive types there, but reached their culmination 



