124 THE STOEY OF THE EARTH AND MAN. 



wliole world. There is mucli in the structure of the 

 leaves of thi3 coal plants^ as well as in the vast amount 

 of carbon which they accumulated in the form of coal, 

 and the characteristics of the animal life of the pe- 

 riod, to indicate, on independent grounds, that the 

 Carboniferous atmosphere differed from that of the 

 present world in this w^ay, or in the presence of more 

 carbonic acid — a substance now existing in the very 

 minute proportion of one- thousandth of the whole 

 by weight, a quantity adapted to the present require- 

 ments of vegetable and animal life, but probably not 

 to those of the Coal period. 



Eeturning from this digression to the forests of 

 the Coal period, we may first notice that which is the 

 most conspicuous and abundant tree in the swampy 

 levels — the Sigillaria or seal-tree, so called from the 

 stamp -like marks left by the fall of its leaves — a plant 

 which has caused much discussion as to its aiB&nities. 

 Some regard it as a gymnosperm, others as a crypto- 

 gam. Most probably we have under this name trees 

 allied in part to both groups, and which, when better 

 known, may bridge over the interval between them. 

 These trees present tall pillar-like trunks, often ribbed 

 vertically with raised bands, and marked with rows 

 of scars left by the fallen leaves. They are sometimes 

 branchless, or divide at top into a few thick limbs, 

 covered with long rigid grass-like foliage. On their 

 branches they bear long slender spikes of fruit, and 

 we may conjecture that quantities of nut-like seeds 

 scattered over the ground around their trunks are 



