ORGANIC DEPENDENCE AND DISEASE 27 
TABLE OF GEOLOGICAL DIVISIONS ADAPTED TO NORTH AMERICA 
ERAS 
AGES 
PERIODS 
CHARACTER OF LIFE 
Psychozoic 
Recent 
Rise of intelligence and age of man. 
Cenozoic 
Quaternary 
Pleistocene 
(Glacial) 
Successive glaciation, wide extinction 
of life through cold, followed by quick 
readjustments and rapid evolution. 
Tertiary 
Pliocene 
Miocene 
Oligocene 
Eocene 
The vertebrate stock approaches phys- 
ical culmination; obscure mammals 
achieve the erect position. The earlier 
mammals are of simple type. 
Cretaceous 
Upper 
Diminutive and primitive marsupial- 
like mammals continue to the close of 
this period and start upon a specific 
upward progress. 
Mesozoic 
The great reptiles are becoming fewer 
Lower after having governed the earth in 
infinite variety. 
Jurassic 
Triassic 
Permian 
Carboniferous 
The reptiles at maximum development. 
In the period of their earlier and more 
plastic expressions birdlike reptiles de- 
veloped and started the race of birds, 
$While their origin-stock is represented 
by the primitive dinosaur reptiles. Here 
are the first traces of the mammal stock. 
Climax of Cryptogamous plants. Land 
reptiles and Amphibians fully estab- 
lished. Stalked Eckinoderms (crinoids) 
at their maximum. 
Culmination of lung and armored fishes 
Devonian 
and primitive sharks. Beginning of 
forests. 
Paleozoic 
Silurian 
Scorpionlike arachnids (Eurypterida) 
at their maximum. 
Ordovician 
Reign of invertebrates of all stocks, 
largely affected by dependence and loss 
of function except in Cephalopod mol- 
Cambrian 
lusks and Crustacea, but locomotive 
independence was more generally re- 
tained in the older faunas. 
Proterozoic 
and 
Archeozoic 
Worms, Radiolaria, Calc-algae, Bacteria. 
