104 EVOLUTION IN THE PAST 



precise conditions under which various forms of hfe then 

 disappeared cannot be ascertained. 

 FLYING Flying hzards were also doomed ; but they showed 

 LIZARDS considerable vitality for a long time. Those of the Jurassic 

 were mostly of small size, but in this Period enormous 

 creatures were navigating the air. Take Pteranodon, for 

 instance. Here was an animal with an expanse of wing of 

 eighteen feet ; and other forms were about with even greater 

 wing-spread. This development had necessitated an excise, 

 which no doubt had been levied on the legs ; for these had 

 become much enfeebled. But however undignified these 

 creatures may have appeared as they shuffled along on land, 

 they must have been grand and graceful objects when sailing 

 in the air. 



The -winged Hzards of earlier times were more or less 

 provided with teeth ; but Pteranodon, and many other 

 Cretaceous forms — especially on North American scenes — 

 were toothless. In this they exhibited a change going on in 

 bird-life. But in their case this breaking away from reptile 

 traditions was of no good augury. Indeed it must be 

 regarded as symptomatic of senility and decay in the no 

 longer youthful Order. 



Notice must now be taken of fishes, which, in addition 

 to numberless commotions and cannibalities amongst them- 

 selves, were victimised by various sea-roving birds and 

 reptiles. But internal dissensions and havoc from without 

 had served in the past to bring out their wonderful adapta- 

 biHties ; and a brisk progress continued. 

 SHARKS Although sharks had undergone some modifications result- 

 ing in the evolution of new forms — some speciaUsed as 

 Porbeagle sharks had now appeared — they were destined to 

 remain in a primitive condition of development. The 

 vertebral column continued of gristly substance, and stiU 

 extended to the extremity of the tail. These features, 

 indeed, have continued until the present day. 



The most notable event in the Cretaceous annals of sharks 

 and closely allied forms was the appearance of saw-fishes 

 (Pristiophorus, Sclerorhynchus) ; but the rostrum-teeth, as 

 might be supposed, were not of so trenchant a character 



