ANIMAL KINGDOM. 147 



stone. There are coral-secreting plants as well as animals; and 

 such plants are called Corallines. There are also microscopic in- 

 fusorial plants ; and those secreting silica — the siliceous infusoria 

 (Diatoms) — are known in later times to have made extensive beds 

 of rocks. Limestone beds have been made from the microscopic 

 Khizopods ; for chalk is largely due to their growth and accumu- 

 lation. And these Rhizopods, although animal, are extremely low 

 in the scale, — little above the spores of sea-weeds : so that, if exist- 

 ing then, they simply foreshadowed the future animal kingdom. 



Whenever the earliest plant, however minute, was created, then 

 the grand idea of life first had expression, and a new line of pro- 

 gress in the earth's history was announced. 



Relations of the North American Azoic to the continent. — 

 The map, fig. 139, cannot be examined without perceiving at once 

 the following striking facts : — 



That the great Azoic area of the continent has (1) its longer leg, 

 B B, parallel approximately to the Rocky Mountains and Pacific ; 

 and (2) its shorter, C C, parallel to the Appalachian Range and the 

 Atlantic ; that — 



(3) The peninsula of Florida is nearly in the course of the Pacific 

 branch, BB; and (4) the Missouri and Arkansas Azoic regions (MA) 

 are nearly in the course of the Atlantic branch, C C. 



(5.) That the northwest side of Lake Superior and the Azoic of 

 the Black Hills lie in the same line. 



Such are some of the structure-lines of the continent in this its 

 early or Azoic state. They are features that were never afterwards 

 effaced : instead of this, they were manifested in every new step 

 in the progress of the continent. 



[From this point the progress of the life of the globe is a pro- 

 minent part of geological history. A brief review of the system 

 of life is therefore here introduced, together with some of the 

 details respecting those of the subdivisions that characterize the 

 Silurian age.} 



1. ANIMAL KINGDOM. 



In the Animal Kingdom there are four Sub-Kingdoms, based on 

 four distinct types of structure, each having its system of subdi- 

 visions of several grades or ranks. These sub-kingdoms are as 

 follow, beginning with the lowest : — 



I. Radiates. — Having a radiate structure, like a flower or a star, 

 internally as well as externally. The animals have a mouth and 



