288 



EVOLUTION 



fern leaves in its strata. Some of these, 

 however, were seed ferns, sometimes 

 considered to be the precnrsors of the 

 flowering plants. In addition, giant, 

 tree-size EquisetumAike plants and 

 others of similar stature, resembling 

 somewhat our modern Lycopodium 

 and Selaginella, flourished in the 

 swamps in Pennsylvanian times. 

 Mosses, liverworts, and the remains of 

 tree-like gymnosperms (in addition to 

 seed ferns) are preserved in Pennsyl- 

 vanian strata. Most of these are still 

 well represented as fossils in Mesozoie 

 strata, but in the Jurassie and Cre- 

 taeeous periods, especially the latter, 

 the angiosperms appeared and became 

 dominant as the number and diversity 

 of other fossils waned. 



Several important generalizations 

 may be made on the basis of this brief 

 survey of the fossil record. 



( 1 ) Indirect and direct evidence 

 indicates that algae, fungi, and bacteria 

 are probably among the most ancient 

 plants, their presence on the earth ex- 

 tending back into the Pre-Cambrian, 

 IVi billion years ago and possibly even 

 longer. Similar organisms, with slight 

 modification, are represented in our 

 flora at the present time. 



(2) Land plants, probably derived 

 from algae which gradually colonized 

 muddy shores and finally drier habitats, 

 had evolved bv the Silurian and be- 

 came more abundant in the Devonian 

 (275 million years ago). 



(3) The widespread occurrence of 

 vascular tissues (xylem and phloem) 

 coincided with colonization of the 

 land. 



(4) Successively more recent strata 

 reveal an apparent orderliness of devel- 

 opment of representative divisions of 

 plants, the order being, from ancient 

 to recent, algae, bacteria, and fungi, 

 other cryptogams and phanerogams. 



Of the latter, the flowering plants are 

 the most recent. 



(5) A number of organisms promi- 

 nent in ancient floras are no longer 

 present in our current floras. In most 

 cases, the reasons for their extinction 

 are not clear. 



Plant fossils, then, indicate that our 

 present flora is changed in composi- 

 tion as compared with floras of earlier 

 periods of the earth's history. Since we 

 know that living organisms are de- 

 scendants of other living precursors, 

 we conclude from the fossil record 

 that our present plants (and animals, 

 of course) are the modified descend- 

 ants of more ancient ones. This, in 

 essence, is what is meant by evolution. 

 All modern biologists accept this point 

 of view. When individual biologists 

 attempt to outline the course of evolu- 

 tion and thus to draw up the actual 

 ph}logenetic lines of descent, espe- 

 cially among the taxa more compre- 

 hensive than genera, they often dis- 

 agree, because individuals interpret 

 evidences differently. 



One who surveys the comparative 

 morphology of living plants in the 

 light of the paleobotanical record usu- 

 allv becomes convinced that terrestrial 

 plants have evolved from aquatic algal 

 precursors, and that the primitive, 

 spore-bearing cryptogams which grew 

 upon the earth from the Devonian 

 through the late Paleozoic periods have 

 now themselves been crowded into near 

 oblivion by the flowering plants that 

 have been dominant since the Cre- 

 taceous. What will occur in plant life 

 in the millions of years ahead is open 

 to speculation. The changes are occur- 

 ring at present, inexorably, but the 

 framework of our human life span 

 clouds our perception of the long-range 

 events yet to transpire in the evolu- 

 tionary process. 



