INTRODUCTION TO THE PROBLEM 



These general questions are, first, to determine whether any of the evolutionary 

 mechanisms shown to be operating in the Cruciferae can also be detected in the 

 Pteridophyta. If this is answered in the affirmative the second question is to inquire 

 whether any further light can be thrown upon such mechanisms by observing the 

 accumulation of their effects in a longer period of geological time than the Flowering 

 Plants have yet had at their disposal. In precise terms this amounts to asking, in the 

 first place, whether the processes of genie mutation, aneuploidy, polyploidy, hybridiza- 

 tion and the rest, have or have not played a recognizable part in species formation in 

 the Pteridophyta; and secondly, to see if any of the predictions aroused by the data 

 already obtained from the Cruciferae can be confirmed or extended by comparison 

 with the older group. Consideration of this second question will be left to the end of the 

 book. The earlier chapters will primarily be devoted to ascertaining the facts relevant 

 to the first, and since the particular evolutionary processes of polyploidy and hybridiza- 

 tion are the easiest to demonstrate, these will be uppermost in mind in the early stages 

 of the inquiry. 



25 



