LINES OF EVOLUTION 165 



likely to make a mistake and assume that a form with 

 this latter type of embryogeny is necessarily related to 

 the one with the more primitive type. The two may 

 or may not be closely related. If related, the one with 

 the more complete segmentation of the egg is the ancestor 

 and the other the offspring, for we could hardly expect 

 reversions of generic rank in genera so widely separated 

 as those of the cycads. 



These illustrations which we have partly recalled from 

 earlier chapters and partly restated, together with others 

 which have been described in various parts of the book, 

 show very clearly that evolution does not progress at 

 equal rates in all the organs of a plant. In Cycas the 

 female sporophylls are quite leaflike, but the male sporo- 

 phylls have lost entirely the pinnule character and have 

 become grouped into a compact cone. On the other 

 hand, in the Mesozoic Cycadeoidea the male sporophylls 

 are leaflike and form a loose crown, while the female 

 sporophylls have lost all resemblance to foliage leaves 

 and are grouped into a cone. Both were doubtless 

 derived from a form in which both female and male 

 sporophylls were leaflike and in loose crowns. Cycade- 

 oidea has retained the primitive male sporophyll, and 

 Cycas the primitive female sporophyll. 



In Dioon the female sporophyll is much more leaf- 

 like than in Encephalartos, but in the embryogeny 

 Encephalartos shows a much more extensive segmenta- 

 tion of the egg. Dioon has retained more persistently 

 the primitive sporophyll character, while Encephalartos 

 has retained the more primitive embryogeny. 



Contrasts are very striking in Microcycas. The male 

 gametophyte is undoubtedly the most primitive yet 



