Seed-Plants 135 



order of mostly tropical ferns, the Marattiaceae, 

 with which there is little question that they are 

 directly related. Others were more like the lowest 

 of the existing seed-plants, the Cycads, and were 

 probably the direct progenitors of the latter. The 

 cycads become more important in the later Paleozoic 

 formations, but reach their maximum development 

 in the next great geological epoch, the Mesozoic. 



The predominance of cycads in the early Meso- 

 zoic suggests that the climate of that period very 

 materially differed from that of the Carboniferous. 

 The rank growth of pteridophytes at that time 

 must have been conditioned by an excess of mois- 

 ture, and a probably very even temperature. The 

 modern cycads are for the most part plants of the 

 sub-tropical and drier tropical regions, where they 

 are usually subjected to more or less extended peri- 

 ods of drought. It may be that increasing dryness 

 was one cause of the tendency to seed-formation in 

 the later Paleozoic time. It is true that there was 

 one group of seed-plants, the Cordaitales, already 

 well developed, which became extinct before the end 

 of the Paleozoic, while, as we know, seeds were de- 

 veloped in various of the fern-like plants and in 

 the club-mosses. But there is some evidence that 

 even during the Paleozoic there were fluctuations 

 in the amount of moisture, and it is possible that 

 these fluctuations may coincide to some extent with 

 the periods of seed formation. 



The cycads have now given place largely to the 



