406 



THE CHANGING GENERATIONS 



fossils of bacteria have also been found in later rocks. The best evidence 

 of the presence and activity of bacteria, however, is that soils formed, 

 dead bodies decayed, and plants flourished in the past as they do today. 

 The Algae rival the bacteria in their claims to antiquity and have 

 greatly influenced the course of geologic processes and the development 

 of life. The blue-greens (Figs. 26.3 and A.l) are the simplest and probably 

 the most ancient; they have many similarities with bacteria, and as 



Fig. 26.5. Some types of modern red algae. 



Fig. 26.6. Some types of modern brown algae, 



previously mentioned, they are believed to have been the principal factor 

 in the formation of the great limestone deposits of Proterozoic (and 

 presumably also of Archeozoic) time. The green algae (Figs. 13.11, 13.12, 

 26.4, and A.2) are more important. Not only are they the probable 

 ancestors of all the higher plants, but they also form a significant part 

 of the drifting plant life of seas and lakes — phytoplankton, which is the 

 basic food supply for all aquatic animals. Some of the green algae play a 

 prominent role in limestone formation, adding to the calcium carbonate 



