
IN NATURAL HISTORY 17 

portant assemblage. So the order Proboscidea contains only two living 
species of elephants, but zoologically it is even more important than the 
Glires, or rodents, which includes the majority of all mammals. 
This classifying of animals may be compared to the organization of 
an army composed of thousands of individuals (species) distinguished as 
officers and privates (genera) formed into companies (families), regiments 
(orders) and brigades (classes) which in turn constitute divisions (phyla), 
the whole vast total forming an army like the-animal kiagdom. 
So the classification of animals is merely an expression of their 
degrees of relationship to one another and enables the naturalist to place 
his species as a general does his soldiers. 
LIFE AND TIME 
The existing plant and animal life of the world is the result of 
evolution through long geologic ages, during which race after race of 
animals came into being, flourished for a time, and wholly or largely 
died out. The table on a following page shows the estimated age 
of the world, the length of different periods in the past during which 
this evolution took place, the predominant life of these periods, and the 
point of origin, so far as known from fossils, when this life began. 
As we are dependent on fossils for our knowledge of the life of the 
past it may be well to devote a few lines to the subject of 
HOW FOSSILS ARE FORMED 
based on Dr. Matthew’s account in the General Guide. A fuller des- 
cription may be found in Animals of the Past. 
In a general way, fossils are the petrified remains of plants or 
animals that lived at some past period of the earth’s history, but they 
include such things as trails left by worms and other creeping things 
and footprints of animals on the sands of time. In many instances we 
have not the objects themselves but only their casts or impressions ‘in 
the rocks. This is particularly the case with shells. Sometimes, as 
with the bones of the great Irish elk, the objects have been buried in 
swamps or bogs, and in a few rare instances, as with the mammoth and 
woolly rhinoceros, entire animals have been preserved for thousands of 
years in ice or frozen mud. Fossils are found in localities where the 
dead animals or plants have gradually been buried under layers of sedi- 
ment to such a depth that they come in contact with the mineral 
waters of the earth and finally become petrified, the essential point 


