772 



CONCLUSION 



xxxii. 6 



fossils found in the Triassic; indeed, all sharks are quite like their 

 Palaeozoic ancestors. 



Sphenodon has changed little since Permian times and hardly at all 

 since the Jurassic, perhaps 140 million years ago. Among mammals, 



FISHES 



FIRST APPEARANCES 

 PER MILLION YEARS 



GENERA 

 •-----• FAMILIES 

 o a ORDERS 



C. F. O. 

 16 1.8.36 



- 16 1.6.32 



S 1 D ' C ' Pi T ' J 

 Geological Periods 



Fig. 513a. Graphs showing the first appearances per million years of known orders, 



families, and genera of the four classes of 'fishes'. O. orders; F. families; and G. 



genera per million years. Time scale is in geological periods. (After Simpson.) 



the opossums and hedgehogs are quite similar to those of Cretaceous 

 times, nearly 100 million years ago, and there are several mammals 

 that have survived with little change for the 50 million years since the 

 Eocene, for instance dogs, pigs, and lemurs. In none of these, however, 

 has a form survived absolutely without change ; they are examples of 

 the persistence of a type of organization rather than of superficial 

 details. 



