42 THE THEORY OF EVOLUTION 



original characters, may well be considered as the most 

 primitive insects, but not as the lowest/ 



(3) The law of the limitation of such specializations 

 in one direction Explanation of the extinction 

 of such forms. 



(a) The almost sudden disappearance of many 

 multifarious groups has long presented a particularly 

 attractive problem. In the Primary period there 

 appeared, for example, Trilobites in the Cambrian 

 system, they passed their prime in the Silurian and 

 Devonian, and entirely disappeared in the Permian. 

 The same thing is observed in the Primary epoch 

 with the Graptolites, Cystoidse and Blastoidse (sea 

 urchins), with the Tetracorals and Euryptidae (gigantic 

 crustaceans). 



In the Secondary period there may be similarly 

 observed the appearance and disappearance of the 

 Belemnites (Thunderbolts), Hippurites, and the gigantic 

 Saurians (Ichthyosaurus, Plesiosaurus, and Pterosaurus). 



In the Tertiary period occurs the appearance and 

 entire extinction of many large groups of Mammalia. 



(b) It has been endeavoured to account for this 

 enigmatical disappearance of entire classes and orders 

 of animals by catastrophes (Cuvier), or also by epidemics 

 and starvation (Neumayer, Quenstedt). 



But the two ' laws ' already discussed afford quite 

 another explanation. 



It had always been noted that ' the species of a 

 group find themselves on the eve of disappearance 



