LINES OF EVOLUTION 167 



Various illustrations could be added showing that 

 evolution along some lines has been more rapid than 

 along others. This is not a new idea, for it is well known 

 that plants show a progressive increase in the complexity 

 of the sporophyte, accompanied by an equally regressive 

 simpHfication of the gametophyte; but the tracing of such 

 lines of evolution should be of the greatest importance in 

 determining relationships. 



In our opinion a cycad in which the embryogeny has 

 progressed so far that there is no complete segmentation 

 of the egg could not give rise to one with such a complete 

 segmentation. This character alone would show that 

 Zamia could not have given rise to Encephalartos. As 

 far as sporophylls and embryogeny are concerned, Cycas 

 might have been the ancestor of Dioon; but the reverse 

 could not have been true, for Dioon has the more ad- 

 vanced sporophyll and the more advanced embryogeny. 

 Of course there is a theoretical possibility that some 

 ancient Dioon with pinnate sporophylls and complete 

 segmentation of the egg may have given rise to Cycas 

 and then proceeded more rapidly than its offspring to 

 reduce its sporophylls and simpHfy its embryogeny; but 

 all available facts indicate that Dioon is more recent. 

 The genus Cycas, as it exists today, could not have been 

 derived from any known Dioon; but Dioon could have 

 arisen through a modification of Cycas. 



The predecessors of the cycad line were ferns, and 

 from the ferns there emerged those primitive seed plants, 

 the Cycadofilicales, which looked like ferns and were 

 long believed to be genuine ferns. There can be no 

 doubt that the Mesozoic Bennettitales came from the 

 Cycadofilicales; but whether the cycads came from the 



