THE HISTORY OF PLANTS 



403 



of protozoa and arthropods are as yet without good foundation. The 

 absence of fossils in the Archeozoic and their great scarcity in the Pro- 

 terozoic rocks has led to the grouping of these two eras into the cryptozoic 

 eon, or age of hidden life, covering the first two-thirds of earth history. 

 Few though the Proterozoic fossils are, they prove not only that life 

 was present but also that evolution was already far advanced before the 

 end of the era. Cell structure had been perfected, animals had become 

 differentiated from plants, multicellular types had evolved from unicellu- 

 lar ones, and at least some of the important groups of plants and animals 



Fig. 26.2. Ancient algae. Natural exposure of a part of a reef of the lime-secreting alga 

 Cryptozoon at Saratoga, N.Y. The Pleistocene glaciers have ground off the surface of these 

 Cambrian rocks, forming natural cross sections of the rounded algal "heads." (Courtesy 

 American Museum of Natural History.) 



had come into existence. Among plants, we know or can reasonably infer 

 the presence of many kinds of bacteria and algae; among animals, mem- 

 bers of the phyla Protozoa, Porifera, Coelenterata, Annelida, Brachiopoda, 

 and Arthropoda. Why, then, are pre-Cambrian fossils so rare? The 

 answer seems to be that few organisms as yet secreted limy or silicious 

 skeletons, and that pre-Cambrian plants lacked cuticle and woody tissues 

 - — in other words, there were no hard parts suitable for fossilization. 



Some biologists, impressed with the high degree of organization shown 

 by the cell, have maintained that evolution from the beginning of life 

 to the perfected cell may have taken as long as all subsequent evolution. 

 Others are of the opinion that it need not have taken long, in the geological 

 sense, for the cellular stage to have been reached. In any event, as soon 



