INTRODUCTION 



pre-Cambrian rocks of Western America a Dumber of forma of 

 these organisms which are supposed to be responsible for extend 



limestone deposits of that region. 



Most of the lower plants, however, like the algae, or Bea-weeds, 

 have very delicate tissues extremely perishable, and capable of 

 being preserved in a fossil state, only under the mosi exceptional 

 conditions; so it is not remarkable that the geological history of 

 these plants is very incomplete, and many of the records of im- 

 pressions of algae are, to say the least, doubtful. 



There is good reason to believe that many of the simplest of 

 the living algae are but little changed from their ancient relath 

 of the early geologic time. We are told that the ancient Beas 

 were fresh-water, and the ancestors of the living plants presumably 

 originated in a fresh-water environment. As conditions in fresh 

 water have not greatly changed, it is reasonable to suppose that 

 as in the case of such simple animals as Amoeba and many flagel- 

 lates, many of these primordial plants have come down to modern 

 times with little change. 



From these primitive green algae it is believed the higher green 

 plants, including the flowering plants, have been derived. 



While most of the green algae have left no recognizable fossil 

 remains, there are some which secrete calcareous incrustation-. 

 which have enabled the geologist to recognize these in a fossil 

 state. The oldest of these go back to the Silurian. Two types 

 especially, Siphoneae now mostly tropical marine algae, and the 

 Charales (Stoneworts), fresh water forms, occur without question 

 in a fossil condition. 



Two very important classes of algae, the red and brown sea- 

 weeds, are probably of more recent origin than the green algae. 

 They are essentially salt-water plants, and it may be assumed that 

 their main characteristics have been developed with the increasing 

 salinity of the oceans in the later geologic time. 



Of the brown algae, the remains are very doubtful in mosi 

 cases, although many fossils have been attributed to this class. 



Among the red algae, there are a good many the >ral- 

 lines"— which secrete lime in quantity, and play a very important 

 role as reef-builders. Corallines are found abundantly in a fossil 

 condition, the oldest fossils attributed to this group occurring in the 

 Silurian; but they are mostly from much more recent formations. 



