300 GENERAL ZOOLOGY 



geology, and in order to do this intelligently we shall have 

 to begin far back, for perhaps more than any other science 

 geology requires of the mind of man vast sweeps of the 

 imagination to form even faint conceptions of the illimitable 

 processes that have brought the earth to its present state. 

 Scarcely less awe-inspiring is the contemplation of the 

 changes that must have taken place in living things since 

 life began in the ocean and on the land. Great as the time 

 and changes were before the earth had life upon it, greater 

 still may be the time that the processes of evolution have 

 required to develop all the forms of life in their complexity. 

 Archaean Time. The earliest period of the earth's geo- 

 logical history is termed the Archaean Era (Gr. archaios, "an- 

 cient"). At first all the substances, including the water which 

 now covers three quarters of the earth's surface, were held 

 suspended in the atmosphere, owing to the high tempera- 

 ture. Later there came a time when the waters condensed, 

 and the surface, cooled still further, permitted the water to 

 cover the rocks of the early crust entirely or in part. Stu- 

 pendous volcanic upheavals must have been frequent as fire 

 and water struggled for the mastery. During this time, of 

 course, no life was possible. As the crust continued to cool 

 it was upheaved and formed land. When the waters had 

 cooled sufficiently to permit of it, life appeared, but in what 

 form we do not know. It is thought that this early life must 

 have been plant rather than animal in its nature. Plants are 

 able, with the exception of the fungi (mushrooms, bacteria, 

 and the like), through their green substance, called chlo- 

 rophyll, to manufacture their food materials out of simple 

 chemical substances. All animals lack this power and con- 

 sequently depend upon green plants to manufacture their 

 food for them. Flesh-eating animals get their food indi- 

 rectly from plants, for the food manufactured by plants 

 is eaten by plant-eating animals, and these in turn become 



