26 THE MORPHOLOGY OF PTERIDOPHYTES 



on this matter. It must always be remembered that our 

 knowledge of past vegetation is based on those fragments 

 of plants that happened to become fossilized and which, 

 furthermore, happen to have been unearthed. It follows, 

 therefore, that a species list will certainly be biassed in favour 

 of plants growing near a particular site of sedimentation and 

 will not give a true picture of the world's vegetation at that 

 time. Thus, until recently, it was thought that the only plants 

 alive in Cambrian times were marine algae and that the land 

 had not yet been colonized. This view would still be held 

 today if macroscopic remains provided the only evidence, 

 but recent discoveries of a wide range of cuticularized spores 

 have shown that there were also numerous land plants in 

 existence. Presumably, they were growing in some habitat 

 where fossilization of their macroscopic remains could not 

 occur. These discoveries of wind-borne spores alter the 

 whole picture of Cambrian vegetation and push further back 

 into antiquity the date of the first colonization of the land by 

 plants. Similar considerations no doubt apply throughout 

 the fossil record to a greater or lesser extent. 



We turn now to the classification of pteridophytes. The 

 first object of any classification must be to group together 

 similar organisms and to separate dissimilar ones. In the 

 process the group is subdivided into smaller groups, each 

 defined so as to encompass the organisms within it. In the 

 early days of taxonomy, when few fossils were known, these 

 definitions were based on living plants. Then, as more and 

 more fossils were discovered, modifications became neces- 

 sary in order to accommodate them, and a number of 

 problems arose. The first arises from the fact that a fossil 

 plant, even when properly reconstructed, is known only at 

 the stage in its life-cycle at which it died. Other stages in its 

 life-cycle, or in its development, may never be discovered. 

 Yet, the classification of living organisms may (and indeed 

 should) be based on all stages of the life-cycle. The second 

 problem concerns the difficulty, when new fossils are dis- 



