Paleophytology. | 419 
found enclosed in amber, and even in diamonds, or have 
been covered up by the hardening mud, and have at least 
left zmpressions in it, from which the form and even the 
species can be recognised. ‘The numberless small cavities 
—cells and intercellular spaces—which are found in the 
substance of the plant are also not unfrequently filled with a 
fluid, the mineral constituents of which—usually silica—be- 
come gradually solid, and thus preserve the structure of the 
plant in a /fossi/ form, the organic parts undergoing decom- 
position. From the mode tn which these impressions and 
fossils are formed, it is evident that they can be found only 
in stratified rocks. ‘The number and systematic position of 
the fossil remains obtained in this manner vary greatly in 
the different superincumbent strata ; but are subject to the 
following general rules :—In the lowermost and consequently 
the oldest strata no remains of plants now living are found. 
In the higher strata are found gradually more and more 
highly organised kinds, and more nearly allied to those at 
present existing; this being most strikingly manifested in 
the uppermost or most recent beds, in which a number of 
fossil plants, or their impressions, are found, which belong 
to our existing flora. If, indeed, an attempt is made to 
form, from the few remains at present found, an idea of the 
entire vegetation—not, perhaps, of the whole earth, but of 
_ those regions which have furnished most material for ex- 
-amination—the result may not differ very widely from the 
reality. It is best to commence with the oldest strata, from 
which a gradual approach may be observed towards the 
existing vegetation of the earth. 
In considering the variety of the fossil remains in the 
different strata, special attention must be paid to the changes 
which have taken place in the climate of the earth in the 
course of the different geological periods.! 
1 [In his Genera et Species Plantarum Fossilium, published in 1850, 
Unger reckoned the number of genera and species of fossil plants known 
up to that time as under :—Exogens, 151 genera, 547 species ; Endo- 
EE2 
