146 The Bible of Nature 



velopment; the sea-serpents and the monstrous 

 terrestrial dragons were no weaklings when death 

 gathered them; the flying reptiles, small dur- 

 ing the Jurassic, attain large dimensions by the 

 end of the Cretaceous, and then pass away 

 forever. 



We cannot do much more than guess as to the 

 conditions of the extinction of races. Sometimes, 

 perhaps, there were changes of environment, to 

 meet which the plasticity of the creatures was in- 

 sufficient; sometimes, perhaps, the struggle for ex- 

 istence was to the death, as it may have been 

 between cuttlefishes and trilobites, between Ich- 

 thyosaurs and Belemnites; sometimes, perhaps, 

 there were constitutional defects, brought about 

 by over-specialization or the like, such as Lu- 

 cretius thought of when he pictured races going 

 down to destruction, " hampered all in their own 

 death-bringing shackles." 



Sluggish sedentary creatures, walled within their 

 castles of indolence, may have become, as it were, 

 smothered in these. This is suggested by the ex- 

 treme calcification of certain extinct types like 

 the Cystoids and Blastoids. Others again, like the 

 flying dragons, have perhaps lived too quickly 

 for their constitutions, life's fitful fever proving 

 too much for them. There seems, also, to be a 

 risk involved in being gigantic or in being very 

 highly specialized. As Marsh says, the Iguanodon 



