24 THE MORPHOLOGY OF PTERIDOPHYTES 



Furthermore, most fossils consist only of fragments of 

 plants. It is then the task of the palaeobotanist to recon- 

 struct, as best he can, from such partly decayed bits, the 

 form, structure and mode of Ufe of the whole plant from 

 which they came. There is small wonder, then, that this has 

 been achieved for very few fossil plants. Many years may 

 elapse before it can be said with any certainty that a particu- 

 lar kind of leaf belonged to a particular kind of stem and, in 

 the meantime, each must be described under a separate 

 generic and specific name. In this way, the palaeobotanist 

 becomes unavoidably encumbered by a multiplicity of such 

 names. 



For convenience of reference, the history of the Earth is 

 divided into four great eras. The first of these, the pre- 

 Cambrian era, ended about 500 milUon years ago and is 

 characterized by the scarcity of fossils, either of animals or of 

 plants. Then came the Palaeozoic era, characterized by 

 marine invertebrates, fishes and amphibians, the Mesozoic 

 by reptiles and ammonites and, finally, the Cainozoic, ex- 

 tending to the present day, characterized by land mammals. 

 These major eras are again divided into periods (systems) 

 and then subdivided again, chiefly on the basis of the fossil 

 animals contained in their strata. While such a scheme is 

 clearly satisfactory to the zoologist, it is less so to the 

 botanist, for the plants at the beginning of one period (e.g. 

 the Lower Carboniferous) are less like those of the end of 

 the period (the Upper Carboniferous) than Hke those of the 

 end of the previous period (the Upper Devonian). Thus, it 

 is more usual for the palaeobotanist to speak of the plants of 

 the Upper Devonian/Lower Carboniferous than of the 

 plants of the Carboniferous period. 



The sequence of the various geological periods is summar- 

 ized as a table (p. 25), in which the time scale is based on in- 

 formation from R. N. C. Bowen^. Brief notes are included 

 to indicate the kind of vegetation that is beheved to have 

 existed during each period, but a word of caution is necessary 



