EVOLUTION-PAST AND PRESENT 



647 



Fig. 25-20. Rates of evolution in several classes of vertebrates are shown from Ordovician to Tertiary. The curves 

 indicate the number of new genera that appeared in each one million year period. The numbers to the left repre- 

 sent the number of new genera. 



genera have originated in several groups 

 of animals from the Cambrian to the Terti- 

 ary (Fig. 25-20). The amphibians start 

 producing new genera at a slow rate during 

 Devonian times and then build up to a 

 peak in the Pennsylvanian. The rate drops 

 off until it reaches its lowest numbers in the 

 Jurassic and Cretaceous, gradually coming 



back in the Tertiary. The reptiles produce 

 new genera at a tremendous rate from their 

 very beginnings in the Pennsylvanian 

 reaching a sharp peak in the Permian, fol- 

 lowed by a sharp decline in the Triassic 

 and levelling off from that point on to the 

 present. Mammals, starting in the Jurassic, 

 show first a decline and then a sudden and 



