GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 221 



remarkable perfection, that thin sections of them may 

 be examined with the microscope and reveal the mi- 

 nutest details of their cellular structure. A careful 

 study of such remains has thrown much light upon the 

 real nature and affinities of many fossil types. Very 

 rarely, it is true, have the reproductive parts, so essen- 

 tial in classification, been preserved; but occasionally 

 this occurs, and a study of these has been of the 

 greatest value in determining the relationship of these 

 fossil forms. 



Unfortunately, too much of the work upon fossil 

 plants has been done by men who were not botanists 

 and who were not sufficiently acquainted with the exist- 

 ing plants allied to the fossil ones. Consequently great 

 confusion has arisen in the attempts to name and classify 

 these fossils. 



In general it may be said that the geological record 

 bears out the conclusions reached from a study of com- 

 parative morphology, although as regards the Thallo- 

 phytes the record is too imperfect to have much value. 



The earliest recognizable plant remains occur in the 

 lower Silurian rocks, where there have been found im- 

 pressions which have been referred to algse, perhaps 

 related to the coarser red or brown forms existing at 

 present, but not readily assignable to any existing 

 types, so that the real nature of these plant remains, if 

 such they really are, is exceedingly doubtful. 



Of the existing types of algas, a number are known 

 in a fossil state, but seldom from the earlier rocks. Of 

 the green algse, certain Siphonese occur fossil in large 

 numbers from the Permian rocks upward. These plants, 

 like many existing ones, were heavily encrusted with 



