74 On the Vegetation of the Carboniferous Period^ 



the whole amount of species in each, no conjecture can be formed, 

 or any but a very rough one of the number into which those with 

 which we are familiar, as of common occurrence, should be divided. 

 The Ferns far out-number, probably, all the others ; but this again 

 materially depends on the value, according to the markings of Si- 

 giUari(B, as means of dividing that genus ; for if the slight differ- 

 ences hitherto employed be insisted upon, the number of the so-called 

 species may be unlimitedly increased. 



2. With regard to the geographical distribution of the species, 

 &c., it appears that an uniformity once existed in the vegetation 

 throughout the extra-tropical countries of the northern hemisphere, 

 to which there is now no parallel ; and this was so, whether we con- 

 sider the coal-plants as representing all the flora of the period, or a 

 part only, consisting of some widely-distributed forms that charac- 

 terised certain local conditions. Nor is this uniformity less conspi- 

 cuous in what may be called the vertical distribution, the fossils in 

 the lowest coal-beds of one field very frequently pervading all the 

 succeeding beds, though so many as thirty may be interposed be- 

 tween the highest and the lowest. 



o 



Of the relations between the soil and the plants nourished by it, 

 little more is recognisable than that the SigillaricB have been parti- 

 cularly abundant on the under-clay, which, judging from the absence 

 of any other fossils but Sigillarise roots (Stigmarice), seems to have 

 been either in itself unfriendly to vegetation, or so placed (perhaps 

 from being submerged) as to be incapable of supporting any other. 

 The latter is the most probable, because both Slgillarice. and their 

 Stigmaria roots occur in other soils besides under-clay, and are there 

 accompanied by Calamites, Ferns, &c. The Coniferce, again, are 

 chiefly found in the sandstones ; and their remains being exceed- 

 ingly rare in the clays, shales, or ironstones, it may be concluded 

 that they never were associated with the Sigillarice and other plants 

 which abound in the coal-seams ; but that they flourished in the 

 neighbourhood, and were at times transported to these localities. 

 The quantity of moisture to which these plants were subjected must 

 remain a question, so long as some authors insist upon the SigillaricB 

 being allied to plants now characteristic of deserts, and others, to 

 such as are the inhabitants of moist and insular climates. The sin- 

 gularly-succulent texture and extraordinary size of both the vascular 

 and cellular tissue of many, possibly indicate a great amount of hu- 

 midity. The question of light and heat involves a yet more import- 

 ant consideration, some of the coal-plants of the arctic regions being 

 considered identical with those of Britain. How these can have ex- 

 isted in that latitude, under the now prevailing distribution of light 

 and heat, has not been hitherto explained ; they are too bulky for 

 comparison with any vegetables inhabiting those regions at the pre- 

 sent time, and of too lax a tissue to admit of a prolonged withdrawal 



