AS COMPARED WITH THAT OF THE PRESENT DAY. 395 



tion of the species, &c. ; their extension over the surface of the country, 

 and the replacement of one kind by another. 3. The relations which 

 may be traced between the species and the soil to which some are 

 peculiar, whilst others are common to it, and to very different soils ; the 

 quantity of moisture, heat, and light they require, and the effects of a 

 diminution or increase of any of these elements. 4. The reciprocal 

 influence of the whole mass of the vegetation upon the surface it 

 covers ; the new soil, or alteration of the old, produced by its decay; 

 the extent and composition of accumulations of dead matter ; the par- 

 ticular kinds of plants contributing most largely to, and the consequent 

 nature of, such deposits ; the conservative influence of the vegetation upon 

 this deposit, which may be retained by roots, and sheltered by foliage 

 from the action of elements, which, in the absence of these protections, 

 would rapidly sweep it away. 



An enumeration of these points, viewed in their bearing on the 

 subject of the Coal-flora, will show how limited is our knowledge of 

 any one of them, compared with what might be acquired from a very 

 superficial examination of any recent flora, or with what the geologist 

 may obtain from an inspection of the animal remains in many strata. 



1. Of the mutual affinities of the groups under which the majority of 

 the genera of coal-plants arrange themselves, little more can be said, 

 than that the Ferns occupy the lower end of the series, and the Conifer® 

 possibly the highest ; but this depends upon the view taken of the 

 affinities of Sigillarice, the most important group. These are classed 

 by some observers amongst Ferns ; by others, with Coniferce ; another 

 considers them as linking these two widely-different families ; whilst a 

 fourth ranks them much higher than either. The affinities of another 

 group, Calamites, are entirely unascertained. Of 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 

 accorded to the markings of Sigillarice, as means of dividing that genus ; 

 for if the slight differences 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 consider the coal plants 

 as representing all the flora of the period, or a part only, consisting of 

 some widely-distributed forms that characterized certain local con- 

 ditions. Nor is this uniformity less conspicuous in what may be called 

 the vertical distribution, the fossils in the lowest coal-beds of one field, 



