THE EVOLUTION AND PHYLOGENY OF THE GROUP 



In any investigation there is a temptation to indulge 

 in speculation and philosophy. I have tried to keep the 

 preceding chapters largely descriptive, thinking it best 



not to mix actual observations 

 with theoretical considerations. 

 In the following pages I have 

 drawn some conclusions which 

 seem to be warranted by the 

 facts, and have ventured 

 cautiously into the domain of 

 theory. 



Since many organs, like the 

 sporophyll, can be traced from 

 the Paleozoic through the 

 Mesozoic, and through the 

 living forms, trends in evolu- 

 tion can be studied with more 

 confidence in the cycads than 

 in groups not favored with 

 such long and well-known 

 geological records. A diagram 

 of the geological horizons, 

 together with the position of the cycads and their 

 ancestors, and also the Bennettitales (the "fossil 

 cycads" of Wieland) will help those who are not 

 famihar with paleobotany (Fig. 78). 



Fig. 78. — Diagram of 

 geological horizons. 



