502 PALAEONTOLOGY OF 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 Gabon iferous 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 Lycopodiaceoun plants, represented by the genera 

 Lepidodendron, Knorria, Ulodendron, Sigillaria, etc., are already represented by 



