404 



THE CHANGING GENERATIONS 



as the development of the cell made cell aggregation and multicellular 

 organisms possible, there is reason to suppose that the pace of evolution 

 increased and that the principal evolutionary lines were not long in 

 appearing. As life has become more abundant and varied, the rate of 

 evolutionary change in most groups seems to have continued to accelerate, 

 through the interaction between gene complexes of increasing evolu- 

 tionary potentialities and environments that became increasingly varied, 

 competitive, and selective. 



The Proterozoic era closed with the second great "revolution"; again 

 the continents rose, the seas withdrew, and the lands were profoundly 

 eroded during a very long interval of which almost no record remains. 



Fig. 26.3. Some types of modern blue-green algae. 



When the seas finally came back over the lands in Cambrian time, their 

 sediments, resting on the truncated surface of the cryptozoic rocks, were 

 filled with a profusion of fossils. This return of the seas marked the start 

 of the Paleozoic era, or age of ancient life — and the real beginning of the 

 fossil record. 



The Cambrian, Ordovician, and Silurian rocks were mostly laid down 

 in the seas, so that the early Paleozoic record is essentially that of marine 

 life. Apparently the lands were dreary, lifeless wastes at least until late 

 Cambrian time and perhaps longer. The seas, however, swarmed with 

 life — with many kinds of animals, among which trilobites and brachiopods 

 were dominant, and what must have been an equal abundance and variety 

 of simple plants. Unfortunately the" plants left fewer fossil remains than 

 did the animals. They lacked woody parts and surface cuticle (the most 

 commonly fossilized parts of plants), while in contrast many of the early 

 Paleozoic animals were well supplied with limy skeletons or armor. The 

 plants were all members of the lowest division, Thallophyta, although 

 all the animal phyla (except perhaps the Chordata) seem to have been 

 present by Cambrian times. Since the fossil record of the thallophytes is 



