PLANTS 



31 



primitive and generalized structure ; but it should not be over- 

 looked that the ability to endure climatic variation may haye 

 been well developed, and supplied a highly important factor 

 in that cosmopolitan distribution which is so sharply marked 

 from the appearance of the earliest land plants down to Upper 

 Jurassic time. 



Preservation of plants. — There are of necessity many gaps 

 in the record left by the vegetation of past ages. The early 

 plants were soft and perishable. Moreover, it is predominantly 

 water life — the life of the sea beaches and the inland lakes and 

 swamps — which has been preserved in the fossil state, since land 

 conditions favor decomposition and destruction and do not fur- 

 nish the conditions necessary to fossilization. It must accord- 

 ingly be only a very incomplete idea of the past vegetation of 

 the earth that fossil plants can furnish. Despite, how^ever, the 

 exigencies of fossilization all the great groups of plants — thal- 

 lophytes (algae, fungi), bryophytes (mosses, liverworts), pteri- 

 dophytes (ferns, horsetails, club-mosses), and spermatophytes 

 (gymnosperms, angiosperms) — have left a fossil record of the 

 larger outlines of their past history. Plant evolution has not 

 been uniform and there are many cases of retrogression, as in the 

 club-mosses and horsetails, yet in general a higher group has 

 succeeded a lower one, the latter not disappearing but falling 

 into the background. Following the early Paleozoic, or age of 

 seaweeds, spore-bearing plants and primitive gymnosperm.s 

 dominated in the Upper Paleozoic. More modern gymno- 

 sperms and the cycadaceous or proangiospermous types made 

 up the still cosmopolitan forest facies of early and mid-Mesozoic 

 time, while from the Cretaceous to the present the angiosperms 

 have been the leading type. 



It must be borne in mind that the study of fossil plants is 

 fully fifty years behind even that of fossil vertebrates. This 

 study, however, is being aided by the discovery of new chemical 

 methods for the examination of carbonized remains and by the 

 appUcation of petrography to this field. Above all, existing 



