THE GEOLOGIC FUNCTIONS OF LIFE. 657 



which, if true, would require the presence of the latter in an early 

 geologic period; but the negative geological evidence relative to their 

 presence favors the alternative view that the Pteridophytes were de- 

 rived from some form of the Thallophytes by an independent line. In 

 recent times, certain of the mosses, especially the sphagnum, mosses, 

 have played a notable part in the formation of peat accumulations. 

 For this, their habit of growing in bogs, and of dying below while they 

 continue to grow above admirably fits them. 



The contribution of the Pteridophytes (ferns, horsetails, lycopods, 

 Sphenophyllum) . — The Pteridophytes include the most important fossil 

 plants of the earlier and middle geologic eras. To them w^e ow^e chiefly 

 the great carbonaceous deposits of the Coal Measures and probably most 

 of the disseminated carbons of the early and middle eras; perhaps also 

 much of the natural oil and gas. Their special work is so conspicuous 

 that it will be noted at length in the chapters on the Devonian and 

 Carboniferous periods, and hence may be passed here with brevity. 

 The ferns, now known more for their beauty than their importance, 

 are the representative type of the group, and are really a wonderful 

 family, having preserved their characteristic leaf-forms with a persist- 

 ence attained by no other group of plants. The Paleozoic ferns are 

 recognizable as such by every one, irrespectiA^e of botanical knowledge; 

 indeed it is the detection of the differences, rather than the resem- 

 blances, between the ancient and modern forms, that requires expert 

 knowledge. This continuity show^s that since their introduction the 

 changes of climate have never been so great as to prevent their propa- 

 gation, without radical modification, in some part of the globe, and this 

 fact rather narrowly limits the range of surface temperatures, and of 

 other climatic vicissitudes. The persistence of the Equisetse (horse- 

 tails, scouring-rushes) and the lycopods (club-mosses) bears hke testi- 

 mony, as does the persistence of life in general; but the rather deli- 

 cate ferns are perhaps more obviously significant than most organisms. 



The contribution of the Spermatophytes (seed plants^ including 

 gymnosperms or " evergreens " and angiosperms or " flowering plants "). 

 — The angiosperms, the dominant group to-day, make their appearance in 

 the record in the latter part of the Mesozoic era, and their contribution 

 is, therefore, relatively modern. They contributed to the coals, lignites, 

 oils, and organic gases of the late geological periods, as did the Pterido- 

 phytes in the earlier periods, the latter participating, however, in the late 



