THE ORIGIN OF MAN 49 



goniatid and ammonid stocks were rapidly chang- 

 ing into a variety of forms. A few words might 

 here be said about the Protozoa whose individual 

 organization is contained in a single cell, and 

 which represent the first form of animal life. 

 These Protozoa were tiny, naked or shelled glob- 

 ules of streaming protoplasm, with a central, 

 more solid sphere known as the nucleus, which is 

 the seat of vital energy or life. They sometimes 

 live singly but more commonly in colonies. 



During the Middle Paleozoic, after the lands 

 had been clothed by plants, a habitat capable of 

 sustaining animal life was thus established and 

 destined to be mastered by several stocks of 

 invertebrates and dominated by vertebrates. 

 Among the backboned animals, the first in- 

 habitants of the dry land were the Amphibia, 

 whose short reign lasted only through the 

 Devonian and Mississippian. Then came the 

 Reptilia, which began their ascendancy in the 

 Pennsylvanian and ruled the lands and even the 



