422 THE CARBONIFEROUS SYSTEM. 



with cross bars — the scalariform vessels. (2.) The Anophytes, or 

 mosses and their allies, with stems and leaves, hut no vessels. (3.) The 

 Thallophytes, or lichens, fungi, sea-weeds, etc., without true stems and 

 leaves. 



In the existing climates of the earth we find these classes of plants 

 variously distributed as to relative numbers. In some, pines predo- 

 minate. In others, palms and tree-ferns form a considerable part of 

 the forest vegetation. In others, the ordinary exogenous trees predo- 

 minate, almost to the exclusion of others. In some Arctic and Alpine 

 regions mosses and lichens prevail. In the Coal period we have found 

 none of the higher Exogens, and only a few obscure indications of the 

 presence of Endogens ; but Gymnosperms abound, and are highly char- 

 acteristic. On the other hand, we have no mosses or lichens, and 

 very few algae, but a great number of ferns and Lycopodiaceas or 

 club -mosses. Thus the Coal formation period is botanically a meet- 

 ing place of the lower Phamogams and the higher Cryptogams, and 

 presents many forms which, when imperfectly known, have puzzled 

 botanists in regard to their position in one or other series. In the 

 present world, the flora most akin to that of the Coal period is that of 

 moist and warm islands in the southern hemisphere. It is not pro- 

 perly a tropical flora, nor is it the flora of a cold region, but rather 

 indicative of a moist and equable climate. Still we must bear in mind 

 that we may often be mistaken in reasoning as to the temperature 

 required by extinct species of plants differing from those now in exist- 

 ence. Farther, we must not assume that the climatal conditions of the 

 northern hemisphere were in the Coal period at all similar to those 

 which now prevail. As Sir Charles Lyell has shown, a less amount 

 of land in the higher latitudes would greatly modify climates, and there 

 is every reason to believe that in the Coal period there was less land 

 than now. Farther, it has been shown by Tyndall that a very small 

 additional amount of carbonic acid in the atmosphere would, by 

 obstructing the radiation of heat from the earth, produce almost the 

 effect of a glass roof or conservatory, extending over the whole world. 

 Again, there is much in the structure of the leaves of the 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 period, 

 to indicate, on independent grounds, that the carboniferous atmosphere 

 differed from that of the present world in this way, or in the presence 

 of more carbonic acid, — a substance now existing in the very minute 

 proportion of less than one-thousandth of the whole, a quantity adapted 

 to the present requirements of vegetable and animal life, but probably 

 not to those of the Coal period. 



