THE ORIGIN AND HISTORY OF ANIMAL LIFE 



617 



THE DISTRIBUTION OF THE FOSSIL REMAINS OF ANIMALS IN THE LAYERS 



OF THE EARTH'S CRUST 



ERAS AND ESTIMATED 

 DURATION OF ERA IN YEARS 



CENOZOIC (Age of 

 Mammals) 



70,000,000 



MESOZOIC (Age of 

 Reptiles ) 



130,000,000 



PALEOZOIC (Age of 



Invertebrates) 



320,000,000 



PERIODS AND EPOCHS 



ANIMALS CHARACTERISTIC OF THE PERIODS 



PROTEROZOIC 

 ARCHEOZOIC 



Quartemary 



Recent 

 Pleistocene 



Tertiary 



Pliocene 



Miocene 



Oligocene 



Eocene 



Paleocene 



Cretaceous 



Jurassic 



Triassic 



Permian 



Carboniferous (Age of 

 Amphibians) 



Devonian (Age of 

 Fishes) 



Silurian (Age of In- 

 vertebrates) 



Ordovician 



Cambrian 



were native here, and some of the finest 

 fossil remains of these ancestors have been 

 found in America. 



The evolution of the horse has been 

 traced back through many distinct stages, 

 extending through the age of mammals and 

 the age of man. A brief description of 5 of 

 these stages, together with Fig. 437, will 

 serve to illustrate the principal changes that 

 took place during this evolution. The struc- 

 tural features that became modified during 



Man; mammals, mostly of species still living. 



Mammals abundant; belonging to numerous ex- 

 tinct families and orders. 



Birdlike reptiles; flying reptiles; toothed birds; first 

 snakes; bony fishes abound; sharks again numer- 

 ous. 



First mammals; birds; giant reptiles; clams and 

 snails abundant. 



Sharks reduced to few forms; bony fishes appear. 



Life transitional between Paleozoic and Mesozoic 

 eras. 



Earliest true reptiles. Amphibians; lungfishes; first 

 crayfishes; insects abundant; spiders; fresh-water 

 mussels; first land shells (snails). 



First amphibians; placoderms; sharks; mollusks abun- 

 dant; first crabs; bony fishes appear. 



First truly terrestrial or air-breathing animals; corals 

 abundant; ostracodernis, placoderms; brachiopods; 

 trilobites; mollusks. 



Ostracoderms, trilobites, cephalopods, brachiopods, 

 and bryozoans. 



Invertebrates onlv. 



Simple marine invertebrates. 



Presumptive origin of life. 



this period were such as to adapt the horse 

 to life on the open plains, where its food 

 consisted of dry grasses. 



The feet of the horse gradually lost the 

 side toes; and only the middle toe and rudi- 

 ments (splints) of the second and fourth 

 digits * remain in our modern horses. The 

 limbs became longer, enabling the animal 

 to move about more rapidly; this change 



* The digits are numbered 1 to 5, beginning on 

 the thumb side. 



