SUBDIVISIONS IN GEOLOGICAL HISTOKY. 



409 



The general facts in the progress of life on the globe are illustrated in 

 the annexed diagram : — 



351. 



ANIMALS. PLANTS. 



Quaternary 



Tertiary, or Era of Mammals 

 Mesozoic, or Era of Reptiles 



Carbonic 



Devonian 



Upper Silurian 



Lower Silurian 



Cambrian 



Archasan 



-^ 



5>^ 



The horizontal bands represent the divisions of time ; the vertical cor- 

 respond to different groups of animals and plants. The lower end of each 

 vertical band marks the point in geological time when, according to present 

 Jcnoidedge from fossils, the type it represents began ; and the varying width 

 in the same bands indicates the greater or less expansion of the type. The 

 following are the points the diagram illustrates : — 



According to present facts from fossils, Radiates began in the Cambrian, 

 and have continued till now, rather increasing throughout the ages. 



Mollusks had their beginning in the Cambrian, and continued increasing 

 to the era of Eeptiles : they then passed their maximum (as indicated in 

 the figure). 



Articulates commenced in the early Cambrian ; and, excluding the tribe 

 to which the Trilobite belongs, they continued expanding in numbers and 

 grade to the present time. 



Wishes began in the Lower Silurian, were very abundant and of great 

 size in the Devonian, and continued on, becoming further diversified in 

 later periods. 



Amphibians began in the Carbonic, and reached their maximum in the 

 early part of the Eeptilian era. 



Eeptiles began in the Permian period of the Carbonic, and had their 

 maximiim in the Eeptilian era or Mesozoic time. 



Mammals began in the Eeptilian era, and became the highest of species 

 in the Mammalian era or Cenozoic time. 



Sea-weeds (or Algge) were the earliest plants of the globe, probably pre- 

 ceding animal life. Acrogens and Conifers began in the Upper Silurian and 

 possibly earlier. The Acrogens had their greatest expansion in the era of 

 Coal-plants, in which they occurred with Conifers. Cycads began in the 

 Devonian, and had their greatest expansion in the Eeptilian era. Angio- 

 sperms or Dicotyledons began in the closing period of the Eeptilian era, 

 and expanded, along with Palms, through the era of Mammals. 



