GEOLOGICAL SYSTEMS AND PERIODS. 7 
of the earth’s history during which organic life existed. Just 
as so-called “ universal history ” falls into larger and smaller 
periods, which are characterized by the oonditions of de- 
velopment of the most important nations at the respective 
epochs, and are separated from one another by great events, 
so we also divide the infinitely longer organic history of the 
earth into a series of greater and less periods. Each of 
these periods is distinguished by a characteristic flora and 
fauna, and by the specially strong development of certain 
vegetable or animal groups, and each is separated from the 
preceding and succeeding period by a striking change in 
the character of its animal and vegetable inhabitants. 
In relation to the following survey of the historical 
course of development which the large animal and vegetable 
tribes have passed through, it will be desirable to say a few 
words first as to the systematic classification of the neptunic 
groups of strata, and the larger and smaller periods corres~ 
ponding to them. As will be seen directly, we are able to 
divide the whole of the sedimentary rocks lying one above 
another into five main groups or periods, each period into 
several subordinate groups of strata or systems, and each 
system of strata again into still smaller groups or forma- 
tions; finally, each formation can again be divided into 
stages or sub-formations, and each of these again into still 
smaller layers or beds. Each of the five great rock-groups 
was deposited during a great division of the earth’s history, 
during a long era or epoch; each system during a shorter 
period ; each formation during a still shorter period. In thus 
reducing the periods of the organic history of the earth, and 
the neptunic strata containing petrifactions deposited during 
those periods, into a connected system, we proceed exactly 
