NATURAL SELECTION 105 



where they prey upon fish. But meanwhile a true land fauna is 

 appearing ; there are small marsupial mammals and archaeopteryx, 

 an indubitable bird though lizard-like. These forms were but 

 crude suggestions of what was to be ; they had none of the per- 

 fection of mechanism afterwards attained by the groups to which 

 they belonged. Such perfection would have been out of place 

 amid the clumsy creatures with whom they shared the world. 

 Tigers would have played havoc among the ponderous Iguano- 

 dons, elephantine reptiles, that appear in cretaceous times. They 

 would have exterminated them and whatever other food supply 

 there was, and would then themselves have starved. A too 

 rapid advance of any one species would have upset the course of 

 evolution. But of such an occurrence there was little danger. 

 For any ground that was gained through lucky variations in 

 individuals could be permanently held, only if there was pressure 

 from competing groups. 



With the Iguanodons come in the Pterodactyls, winged 

 reptiles, some small, some big, their wings measuring twenty- 

 five feet from tip to tip, great expanses of membrane suggesting 

 the wings of bats ; their hollow bones make it probable that they 

 were warm-blooded. What havoc these winged creatures must 

 have made among Iguanodons, even the armoured forms ! The 

 period of quickness in attack and flight now begins on land ; 

 there too, mobility is the all-important thing ; long ago there 

 had been speed in fish and their pursuers. Now, whether on 

 the ground or in air, there is no safety for the slow. And so at 

 the beginning of the Tertiary period, quickly moving mammals 

 become the dominant type. In spite of their wings the Ptero- 

 dactyls prove too clumsy they are clumsy on the ground, and 

 flight alone is not enough and the feathered flyers, active on 

 their legs and not only on their wings, by gradual steps proceed 

 to the most exquisite adaptations. If the mammals are dominant, 

 birds have a rival kingdom and they are often able to invade the 

 territory of their superiors and prey upon them. Among the 

 mammals we see the striking forms of the monster Mastodon ; 

 Dryopithecus, an ally of the chimpanzee, is on the stage, and 

 there is the plainly recognisable ancestor of the horse. These 



