THE LIFE OF THE SIMPLEST ANIMALS 23 



That the character of the body structure of the Protozoa 

 has changed but little since early geologic times is ex- 

 plained by the even, unchanging character of their sur- 

 roundings. The oceans of former ages have undoubtedly 

 been essentially like the oceans of to-day not in extent 

 and position, but in their character of place of habitation 

 for animals. The environment is so simple and uniform 

 that there is little demand for diversity of habits and conse- 

 quent diversity of body structure. Where life is easy there 

 is no necessity for complex structure or complicated habits 

 of living. So the simplest animals, unseen by us, and so 

 inferior to us in elaborateness of body structure and habit, 

 swarm in countless hordes in all the oceans and rivers and 

 lakes, and live successfully their simple lives. 



