4 
502 
PALAEONTOLOGY OP ILLINOIS. 
contrary, Pecopteris unita and Pecopteris plumosa are common at Duquoin and 
Morris, especially in the nodules of Mazon creek, and rare, or not recognized 
as yet at Colchester and Murphysborough. 
From the examination of the table, one may easily see other points of differ- 
ence between the species found at the same station, or of analogy between 
those of different horizons. Nevertheless, I am not, on this account, prepared 
to abandon, as an unsustainable hypothesis, the question of the stratigraphical 
distribution of the fossil plants of the coal, for the following reasons : 
1st. In a theoretical point of view, it is scarcely admissible that at an epoch 
where the land surface has been universally, and at repeated times, modified 
by deposits, either of sand or of limestone, sometimes of great thickness, indi¬ 
cating a prolonged submersion, the flora, re-appearing after these terms of sub¬ 
sidence, has always been represented by the same species distributed in the 
same proportion. Atmospherical circumstances, indeed, are the essential 
agents in modifying the characters of a flora, and these circumstances have 
been apparently the same during the whole duration of the Carboniferous 
epoch. But the elements or components of the soil, or of the water where the 
plants have lived, have been evidently modified at different times, and even if 
the medium affording life to the vegetation had been repeatedly the same, 
some species of plants should have been lost or have somewhat changed their 
forms in these repeated and prolonged submersions of the whole surface of the 
coal fields. The destruction, or the first appearance of a species, either animal 
or vegetable, is the most difficult phenomenon to ascertain. Animal species, 
for example, seem to appear at once, and of far different kinds, in successive 
geological strata. But these strata are either composed of different materials, 
or have been formed in water of various depths, and under other varied cir¬ 
cumstances. The changes of life, therefore, are local or casual phenomena, 
which generally represent a mere displacement of groups, and are of no account 
whatever in considering the first appearance, or the destruction of a single 
species. 
2d. The fossil plants hitherto obtained from the Coal Measures of Illinois 
are mainly the result of local researches, too limited to serve as a basis for gen¬ 
eral conclusions, and it is only after more extended examinations, and more 
complete collections from other portions of the great area now occupied by 
Carboniferous strata in this and the adjacent States, that we may expect to 
obtain the data for determining, in a satisfactory manner, the distribution of 
the Caboniferous flora over the whole extent of our American coal fields. 
3d. When this is done, we shall have sufficient proofs of a^gradual change 
in the characters of the vegetation of the Coal Measures from the first appear¬ 
ance of land vegetation. The Lycopodiaceous plants, represented by the genera- 
Lepidodendron, Knorria, Ulodendron, Sigillaria, etc., are already represented by 
