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 alge, 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 alge, a number are known 
in a fossil state, but seldom from the earlier rocks. Of 
the green algze, certain Siphonez occur fossil in large 
numbers from the Permian rocks upward. These plants, 
like many existing ones, were heavily encrusted with 
