380 PALAEOZOIC SYSTEM OF ROCKS. 



3. The uniformity is proved by the great resemblance and often 

 identity of the species in the most widely-separated regions. Accord- 

 ing to Lesquereux, out of 434 American and 440 European species, 176 

 are common, and the remainder far less diverse in character than the 

 species of the two floras at present. Again, in all latitudes, from the 

 tropics to 75° north latitude, Coal species are extremely similar. Such 

 uniformity of vegetation shows a remarkable uniformity of climate. 

 From the earliest times until the present there has been probably a 

 gradual evolution of continents — a gradual differentiation of land and 

 water, a consequent differentiation of climates, and a corresponding 

 differentiation of faunas and floras. 



4. The carbonated condition of the atmosphere is proved by the 

 large quantity of carbon laid up in the form of coal, the whole of which 

 was withdrawn from the atmosphere in the form of carbonic acid. It 

 is also indicated by the nature and the luxuriance- of the vegetation. 

 The proportion of carbonic acid in the atmosphere is now about -£$ per 

 cent (g^po). Now, since carbonic acid is the necessary food of plants, 

 it is natural to expect that up to a certain limit the increase of atmos- 

 pheric carbonic acid would increase the luxuriance of vegetation. Ex- 

 periments by Daubeny * prove that this is true especially for vascular 

 Cryptogams. 



We may therefore picture to ourselves the climate of this period as 

 warm, moist, uniform, stagnant (for currents of air are determined by 

 difference of temperature), and stifling, from the abundance of carbonic 

 acid. Such physical conditions are extremely favorable to vegetation, 

 but unfavorable to the higher forms of animal life. 



Cause of this Climate. — The moisture and uniformity were the 

 necessary result of the physical geography already given. They were 

 due to the wide extent of ocean and the absence of large continents and 

 high mountains. High mountains are the precipitating points for the 

 atmosphere — points through which it discharges its superabundant 

 moisture. As these did not exist, the atmosphere was always highly 

 charged. The prevalence of the ocean also, as is well known, produces 

 uniformity. 



The greater warmth of high latitudes is partly explained by the 

 uniformity. But there is good reason to believe that there was then 

 a higher mean temperature than now exists. This was probably due to 

 the constitution of the atmosphere. This may be shown as follows : 



The surf ace- temperature of the earth is now almost wholly due to 

 external, not to internal causes. It has been calculated that only one 

 twentieth of a degree Fahr. is now due to the latter cause. In going 

 downward the heat increases about 1° Fahr. for every 50 to 60 feet, 



* Report of British Association for 1849, p. 62, and 1850, p. 159. 



