CARBONIFEROUS PERIOD. 363 



carbonic acid would be injurious to animal life. To vegetable life, 

 on the contrary, it would be, within certain limits, promotive of 

 growth ; for plants live mainly by means of the carbonic acid they 

 receive through their leaves. The carbon they contain comes 

 principally from the air. 



This being so, it follows, as has been well argued, that the carbon 

 which is now coal, and was once in plants of different kinds, has 

 come from the atmosphere, and therefore the atmosphere now 

 contains less carbonic acid than it did at the beginning of the Car- 

 boniferous, by the amount stowed away in the coal of the globe. 



Such an atmosphere, containing an excess of carbonic acid as well 

 as of moisture, would have greater density than the present : conse- 

 quently, it would (1) have increased heat at the earth's surface, and 

 this would be the cause of a higher temperature over the globe 

 than the present. (E. B. Hunt.) This density would (2) tend to 

 diminish the rate of movement in the atmospheric circulation, 

 and the frequency of storms or violent disturbances. 



During the progress of the Carboniferous period, there was, then, 

 (1) a using up and storing away of the carbon of the superfluous 

 carbonic acid, and. thereby, (2) a more or less perfect purification 

 of the atmosphere and diminution of its density. In earlier time 

 there had been no aerial animal life on the earth ; and as late 

 as the Carboniferous period there were only reptiles, insects, and 

 pulmonate mollusks. The cold-blooded reptiles, of low order of 

 vital activity, correspond with these conditions of the atmosphere. 

 The after-ages show an increasing elevation of grade and variety 

 in the living species of the land. 



(4.) Influence of the climate on the growth of plants. — A moist warm 

 climate produces exuberant growth in plants that are fitted for it. 

 The plants of the Coal period were made for the x>eriod. The Sigil- 

 lariai and Calamiies manifest, by their characters and mode of occur- 

 rence, that they could flourish only in a moist region ; and the 

 ferns of the tropics, as well as Equiseta everywhere else, like moist 

 woods. The Lepidodendra, by their association with the Sigillarice and 

 Ferns, show that the same conditions (as is now the case with their 

 kin the Lycopoclia) favored their development. In fact, Lycopodia, 

 Equiseta, and most ferns, are plants that like shady as well as 

 moist places. Adding, then, the prevalent moisture and warmth 

 to the excess of carbonic acid in the atmosphere, we should be 

 warranted in concluding that, even if there was less sunshine than 

 at the present time, vegetable growth must have been more exube- 

 rant than now, especially in our colder temperate zones. This 

 exuberance would not have shown itself in thick rings of growth 



