EVOLUTION-PAST AND PRESENT 



631 





RECEfOT 



TERTIARV 



CRETACEOUS 



JURASSIC 



TRIftSSIC 



PERSIAN) 



PENNSYLVAWIAN) 



niSSlSSIPPIAN) 



DEVJOrOlAM 



5ILURIAK> 



OROOVIClAlv^ 



Fi9. 25-8. The historical record of the vertebrates is shown, where the width of the black pathways indicates the 



relative variety of known forms in the various vertebrate classes. 



a feature that was retained in all subse- 

 quent forms. They also possessed heavy 

 bony skeletons. Both the cartilaginous and 

 bony fishes were offshoots from the placo- 

 derms, the former sacrificino; bone for 

 cartilage and the latter retaining the bone. 

 Today we have both represented: the carti- 

 laginous fishes in the sharks and rays, and 

 the more successful bony fishes which 

 abound in all waters of the globe. The carti- 

 laginous fishes have held their own rather 

 well through this long geologic time while 

 the bony fishes have increased at a tre- 

 mendous rate, particularly since the Per- 

 mian. 



The most primitive land-dwellers, the 

 amphibians, arose from the bony fishes 

 some time during the Devonian, reaching a 

 peak which they maintained through the 

 Pennsylvanian and Permian, but then sud- 

 denly lost ground probably because their 

 chief competitors, the great reptiles, be- 

 came the dominant land life. With the pass 

 ing of the great dinosaurs, the amphibians 

 made a slight comeback which they have 



maintained up to the present. The reptiles 

 came from the amphibians in the Pennsyl- 

 vanian and very soon reached a peak which 

 they continued to hold, with a setback dur- 

 ing the Triassic, until the end of the Meso- 

 zoic (beginning of the Tertiary), when all 

 of the large dinosaurs suddenly disap- 

 peared leaving only remnants which have 

 continued on to the present. The reptiles 

 have been the most successful of all animals 

 living in the past and it is problematical 

 whether any group in the future will reach 

 such peaks as they did. 



Both the mammals and birds were de- 

 rived from the reptiles some time during 

 the Triassic but good records are available 

 only in the early Tertiary. Both started out 

 very slowly, seeming to "bide their time" 

 while the great reptiles ruled tlie earth. 

 With the decline of these beasts, both the 

 birds and mammals came into their own, 

 increasing in numbers and variety at a 

 tremendous rate. One can imagine that 

 through the Triassic and Jurassic small 

 birds and mammals occupied the secluded 



