774 



CONCLUSION 



xxxn. 7- 



7. Varying rates of evolutionary changes 



Although gradual modification of living organization is almost uni- 

 versal it is clear that the change is often extremely slow. The transition 

 from an osteolepid fish, say *Sauripterus } through stages like *Eogyrinns 

 to *Seymonria took nearly 90 million years. The change of the horses 

 from *Hyracotheruim to Equus is not very profound, considering that 

 it took at least 50 million years. Probably few populations stay the 

 same for long periods, but there may be marked differences in rate 

 of change in groups not otherwise dissimilar. Thus some lines of 



"FISHES" FIRST AND LAST APPEARANCES 

 ORDERS PER MILLION YEARS 



TETRAPODS 



S'D'C'P'T'J'C 



Geological Periods 



Fig. 513&. Graphs showing first and last appearances of orders per 

 million years in 'fishes' and in tetrapods. (After Simpson.) 



elephant shortened the lower jaw earlier and faster than others (p. 716). 

 Whereas the majority of elephant populations changed greatly between 

 the Oligocene and the Pliocene, the deinotheres remained almost the 

 same throughout this long period. 



Taxonomic methods show that for each class of vertebrates the rate 

 of diversification increases rapidly shortly after the origin of a class 

 and thereafter falls though sometimes showing a second rise (Fig. 5 1 30). 

 The maximum rate of formation of new orders precedes that of forma- 

 tion of new genera by 25-50 million years in each case. Rates of first 

 and last appearances follow each other rather closely (Fig. 513&) sug- 

 gesting continuous replacement in the populations. 



8. Vertebrates that have disappeared 



Set against the few examples of relative constancy of organization 

 there are the wholesale extinctions that we deduce from study of the 

 rocks. The ostracoderms, osteolepids, stegocephalians, dinosaurs, and 

 pterodactyls (to mention only a few) became completely extinct or 



