AS COMPARED WITH THAT OF THE PRESENT DAY. 391 



supposed species were founded. Again, as the specific characters used 

 in dividing this genus are drawn from what are considered very unim- 

 portant features in recent plants, namely the scars left by the fallen 

 leaves, it is evident that several distinct species may be merged into one, 

 in the absence of other distinctions beyond that solitary character which 

 does not suffice to recognize analogously marked living vegetables. 



The last obstacle which demands a passing allusion, because tending 

 to retard our knowledge of coal fossils, is, that they cannot be investi- 

 gated independently. Representing the earliest known Flora, the 

 individuals composing it are, as might be expected, more unlike those 

 now living, than what any subsequent formation contains. The suc- 

 ceeding beds present us with plants, which occupy, in point of organi- 

 zation, as in date of creation, a middle position ; and it is in many 

 cases through the investigation of these alone that a clue can be gained 

 to the relationship existing between the earliest known and the now 

 living vegetable forms. It is not so to an equal extent in the animal 

 kingdom. A knowledge of recent shells, for instance, can be brought 

 to bear upon those of the Silurian formation (independently of any study 

 of their allies in the more modern strata) far more effectually than an 

 equal acquaintance with living plants can, upon those preserved in our 

 coal-fields. Many and sufficiently obvious are the reasons for this : 

 the Silurian rocks contain but one or few orders of animals, the car- 

 boniferous many of plants. There is a greater external similarity 

 between the shells of all periods — they are better preserved, and their 

 external characters afford surer indications of their affinities, habits, 

 and localities. 



An examination of the coal vegetation being merely a comparison of 

 its tribes of plants with those we are more familiar with, the first object 

 of the naturalist is, to reduce all the strange individual forms he here 

 for the first time sees, to the same classes and orders with existing 

 ones. When their affinities cannot be traced, he seeks to ally them to 

 living analogues, and thus, reproducing the whole Flora, he regards it 

 as probably characteristic of such physical features of soil, surface, and 

 climate, as accompany what he has determined to be the existing 

 types of the by- gone Flora. The general laws now affecting vege- 

 table life, are the only ones available in this comparison, and therefore 

 are adopted as correct ; but to appreciate the extent of their application, 

 a very comprehensive knowledge of the distribution of plants is 

 necessary. Slight local causes may very materially modify the opera- 

 tion of these laws ; and so plastic is vegetation under their influence, 

 that we find what appear to be entirely analogous positions with re- 

 gard to heat, light, soil, and moisture, tenanted by whole genera, and 

 even orders of plants, of very opposite botanical characters, and that 



