366 On the Vegetation of the Carboniferous Period, 



reasons for this ; the Silurian rocks contain but one or few orders of 

 animals, the carboniferous many of plants. There is a greater ex- 

 ternal siniilai'ity 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 bygone Flora. The general laws now 

 affecting vegetable life are the only ones available in this compari- 

 son, 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 operation of these laws ; and so plastic is ve- 

 getation under their influence, that we find what appear to be entirely 

 analogous positions with regard to heat, light, soil, and moisture, 

 tenanted by whole genera, and even orders of plants, of very oppo- 

 site botanical characters, and that such localities present a greater 

 disparity of vegetation than do other countries more remote in geo- 

 graphical position, and with less similarity in their conditions. 



It is the case with very many species of existing plants, that they 

 vary so considei'ably at various parts of the area over which they are 

 dispersed, as to draw all but those who know the intermediate links, 

 which may be comparatively scarce, into a belief that the extreme 

 varieties are specifically distinct. This is eminently true of ferns, 

 which have very wide ranges, and are exceedingly sportive. If the 

 difficulty be great with living plants, of which complete specimens 

 or whole individuals are procurable, it must be far more so with 

 fragmentary fossils ; and the coal formation being characterised by 

 ferns to a very remarkable degree, it follows, that, with only imper- 

 fect specimens, all attempts towards determining the species and 

 limiting them must be rery vague. The amount of variation also is 

 fluctuating, and it bears no necessary reference to botanical affinity ; 

 for, whilst nine species of a genus may be constant to their charac- 

 ters wherever they occur, a tenth may vary so widely that its ex- 

 tremes will appear far more dissimilar than are any two of the other 

 nine. 



The knowledge of recent botany, which is needful to throw light 

 upon the study of fossil plants and the origin of coal, must be both 

 varied and extended, though a profound acquaintance with any par- 

 ticular branch is not required to make a very considerable progress. 

 Those points with which the student should -be most familiar are 



