184 



STUDIES IN EVOLUTION 



but all the other divisions apparently have continued to 

 increase since their inception during Paleozoic time. The 

 only known arthropod contemporaries of the trilobites in the 

 Cambrian are the Merostomata, Ostracoda, Phyllopoda, and 

 Phyllocarida, all of the higher forms apparently having 

 developed since that time. A more graphic view of the 

 geological range and distribution of the arthropods is repre- 

 sented in the following table : — 



84 



Cenozoic 



Mesozoic 



Carboniferous 



Devonian 



Silurian 



Ordovician 



Cambrian 



Pre-Cambrian 



r2 oo 





-^ c3 



P-. Ph 



o 



p^ ^ 



Figure 84. — Geological Range and Distribution of Arthropoda. 



Having thus far reviewed the features of the primitive pro- 

 taspis and some of the characters it acquired through earlier 

 inheritance, together with the comparative age of the differ- 

 ent groups of arthropods, it must be conceded that, in inter- 

 preting crustacean phylogeny from the facts of ontogeny, the 

 trilobites, so far as they show structure, are entitled to first 

 place. Moreover, since the appendages are quite fully known 

 and from them the trilobite proves to be a most generalized 

 and primitive crustacean, still greater reliance can be placed 



