THE NEW EVOLUTION 



Lower Cambrian. They increased markedly in num- 

 bers in the Middle Cambrian and reached a maximum 

 of diversity in the Ordovician and Silurian from which 

 periods we know more than 3,000 different kinds. 

 They continued plentiful in the Devonian and also 

 in the Carboniferous, but toward the end of the latter 

 period they began to decline and since the end of the 

 age of reptiles (Cretaceous) they have been of small 

 importance. 



This parade of facts showing the very different 

 balance of life upon the earth in the distant past might 

 be indefinitely extended. But we have said enough 

 to show that in the different periods and the different 

 eras the continuous changes affected only the forms 

 within each phylum, and never the interrelationships 

 between the phyla. 



Most interesting in this connection are the ammo- 

 nites, for they begin as ammonites, become enor- 

 mously diversified, and then disappear without ever 

 being anything but ammonites. Here we have a 

 group, a division of the cephalopod mollusks allied 

 to the nautiloids, of which the entire history seems 

 to be laid bare for our inspection. The early portion 

 of the history of the ammonites is not lost in the 

 unknown pre-Cambrian as in the case of the trilobites, 

 the eurypterids and the lamp-shells. 



This constant change from age to age involving the 

 animal types within each major group or phylum com- 

 bined with the unchanging constancy of those broader 

 features of animal life by which the members of each 

 phylum are distinguished from the members of all the 



[ill] 



