392 VEGETATION OF THE CARBONIFEROUS PERIOD 



such localities present a greater disparity of vegetation than do other 

 countries more remote in geographical position, and with less simi- 

 larity in their conditions. 



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

 vary so considerably 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 indi- 

 viduals are procurable, it must be far more so with fragmentary fossils ; 

 and the coal formation being characterized by ferns to a very remark- 

 able degree, it follows, that with only imperfect specimens, all attempts 

 towards determining the species and limiting them must be very 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 characters wherever they occur, a tenth may vary so 

 -widely that its extremes 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 some 

 of them purely botanical ; whilst others are more general, and refer to 

 the dependence of vegetation upon the condition of the area it covers. 



Some acquaintance with systematic botany is the first requisite : 

 through this alone can any approximation to the living affinities of the 

 fossil be obtained. It should embrace not only a knowledge of the 

 principal groups or natural order under which all plants are arranged, 

 but a familiarity with vegetable anatomy ; for when the stem or trunk 

 alone is preserved, which is often the case, a minute examination of 

 its tissues is the sole method of determining its position in the natural 

 series. 



A solution of the difficulties which this special knowledge will tend 

 to remove is of the highest interest to botanists, though comparatively 

 preliminary to the object of the geologist, whose inquiry is, what were the 

 general features of such a vegetation as has effected the formation of a 

 seam of coal, both as regards quantity and kind. As regards quantity, 

 inasmuch as the growth was either wonderfully rapid, or more tardy, 

 but prolonged under uniform conditions ; and as regards kind, from cer- 

 tain species, genera, or orders, being particularly adapted by their 

 quick growth, their gregarious habits, and their continued appropri- 



