Evolution of Verjetal Life. 127 



are preserved there are the widest gaps. They were usually 

 laid down upon sea-bottoms, or in shallow lakes, or at the 

 mouths of rivers, and vast intervals of upheaval occurred 

 between them. There were also great changes in temper- 

 ature, and by climatic influences, and the varying connec- 

 tions of islands and continents, alternately elevated and de- 

 pressed, tribes have been pent within narrow limits, or 

 spread to the four quarters of the globe. Nevertheless, in 

 general, the story that geology tells is the stoiy that we 

 should expect to hear. In the several great geologic pe- 

 riods, evidences of the vegetable life of which have been 

 preserved, certain plant-forms have dominated in turn, and 

 in the order of their complexity as I have already defined 

 them. 



First appeared the algse, in the Primordial Epoch, and 

 with them, apparently, no other. In the Primary Epoch, 

 in which were made the great coal deposits of the Carbonif- 

 erous Period, there was an enormous development of mosses, 

 of lycopodiums, of equisetae ^r horsetails, and of giant 

 ferns in great variety and of great beauty. So far as many 

 of these exist to-day, they are characteristic growths of 

 warm countries. It is noteworthy that plant-life was the 

 dominant life of the Carboniferous Period ; that plants grow 

 most luxuriantly in an atmosphere containing an excess of 

 carbonic-acid gas ; that the effect of such an atmosphere 

 would be to greatly raise the temperature of the surface of 

 the globe ; and that during the Carboniferous Period, this 

 tropical vegetation seems to have been spread throughout 

 the circum-polar regions. If, then, at this period, there was 

 an excess of carbonic-acid gas in the earth's atmosphere, 

 all the facts which we note are harmoniously accounted for. 



The latter part of the Primary Epoch and the beginning 

 of the Secondary, saw the development and reign of tlie 

 palm-ferns and the pines ; and then the palms and similar 

 interior growing plants ; and Anally, in the Tertiary and 

 Modern Epochs, became dominant the hard wood trees and 

 varied flora that now in groat part form the royal fittings of 

 the temple of nature in which we dwell. Of old lierbaceous 

 forms the remains are, naturally, relatively few, because of 

 the softness of their substance ; but so far as they appear, 

 their character corres])onds with that of the general record. 



I do not mean that all of our present forms are very re- 

 cent. On the contrary, thougli nut among the earliest, some 



