J. D. Tothili — The Ancestry of Insects. 



385 



From the point of view of vertical distribution a glance at 

 figure 8 shows the possibility of the derivations suggested. 

 Trilobites were well differentiated at the base of the Cambrian 

 series and reached their maximum deployment in Ordovician 

 waters ; they disappeared toward the end of Paleozoic time. 

 The Chilopoda are known from Pennsylvania!! rocks through 



Fig. 8. 





o 

 J 



f- 



O 

 P- 



o 



j 



o 

 p- 



3: 







[s. 



a 



HOLOCENC 













<J 



, "PLEISTOCENE 









o 



2< 



5 



< 



OJ 



"PLIOCENE 









2 



MIOCENE 









%J 



OU&OCENE 











.EOCENE 











tRETACEUS 









o 



COMANCHtAN 









o 



•SI 



-JURASSIC 









Z 



T RIASSIC 











VER^\AN 



1 







cJ 



PENNSYLVANIA^ 



1 







O 



MISStSSIPPlAN 



1 









2 < 



DEVONIAN 



1 





' 





< 



SILURIAN 



A 









a. 



ORDOVICIAN 



■ 



V 









.CANlSRIAN 



W 









TWE 



C ANTBMAN 











Fig. 8. Vertical distribution of trilobites, centipedes and insects. (Original.) 



the discovery of Palwocampa. As this is clearly a highly 

 specialized form (in virtue of the elaborate system of 

 macrochaetae and the small number of segments) it seems prob- 

 able that the group was highly differentiated in Pennsylvanian 

 times and that it arose in much earlier Paleozoic times. Many 

 insects are known from the Pennsylvanian rocks ; it seems 



