THE HISTORY OF PLANT LIFE. 31 



good reason for the belief that the various forms of carbon now 

 found in the crust of the earth must have been derived in the 

 first instance from the atmosphere through the agency of green 

 plants. Sir William Dawson has also expressed the opinion that 

 if the Laurentian graphite may be taken as representing former 

 plant life, it indicates its occurrence in vast profusion, and this 

 was no doubt the case. To these views we may also add the 

 opinion of the late Sterry Hunt, that the great Laurentian 

 beds of iron ore had their origin in plant decay, precisely as 

 they are formed at the present day. 



During those early times the water resting upon the earth's 

 surface was of a rather high temperature. At the same time 

 the atmosphere was charged with a relatively high percentage 

 of carbon dioxide, in consequence of which its temperature was 

 considerably higher than at present. These considerations 

 lead to the conclusion that the earlier and aquatic vegetation 

 of the general character of the Chlorophyceae with which we 

 are familiar to-day, but whose delicate structure did not admit 

 of permanent preservation flourished under conditions now 

 fairly represented by hot springs, where vegetation thrives at 

 temperatures upwards of 93 C. If, furthermore, plants began 

 to emerge from their aquatic condition in later Eozoic time, 

 they must have been brought under an environment essentially 

 similar to that of the tropics of to-day. Certainly this was the 

 case until late in Palaeozoic time. 



We are thus led to the belief that the flora of Eozoic time 

 was not only very abundant, but that it consisted, perhaps 

 wholly, of aquatic thallophytes comparable with the marine and 

 fresh-water algae of to-day ; that those plants were capable of 

 photosynthesis, and that along certain lines they attained a 

 somewhat high degree of development. 



One cannot fail to remark upon the peculiar absence of plant 

 life during Huronian and Cambrian times, while evidences of 

 animal life are abundant. This may have its explanation in 

 the complete destruction of the more perishable plant struc- 

 tures, consequent upon the great disturbances which character- 

 ized the close of the Eozoic and the opening of the Palaeozoic 

 period. That plant life must have been maintained in full 



