EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 407 



tions of relationship, especially among conifers, on the basis of certain 

 features of the female gametophyte, for example, features which 

 represent a stage in a general change that may occur in a number of 

 independent lines. 



In passing from the ancient to the modern gymnosperms, it becomes 

 evident that groujjs differ as to the rapidity with which they respond 

 to a general tendency to change, and it is this difference that helps 

 to constitute groups. A modern group, for example, may associate 

 a number of ancient features with others that are recent; or all of 

 the ancient features may have been changed. This has been called 

 the "lagging behind" of certain structures, but this should not imply 

 that they are held back and will come forward later; it simply means 

 that for some reason they have not responded to the general tendency 

 among gymnosperms to change in a certain direction. The retention 

 of an old structure must not be confused with the reappearance of an 

 old structure. For example, it seems clear that the most ancient 

 gymnosperms were large-leaved forms, from which the small-leaved 

 conifers were derived; and yet small-leaved. pteridophytes may have 

 been more ancient than large-leaved ones. If this be true, the appear- 

 ance of small leaves among conifers is the reappearance of an ancient 

 feature, and not its retention. To prove the retention of an ancient 

 feature demands the establishment of its phylogenetic continuity. 



PHYLOGENY 



Before sketching the general evolutionary tendencies among 

 gymnosperms, it will be of service to outline what seems to be a rea- 

 sonable conclusion as to the phylogenetic connections of their groups. 

 This statement and those that follow will be all the more clear if 

 freed from the details upon which they are based and also from cita- 

 tions. Those who know the facts and the investigators do not need 

 their recital; and those who do not know them would only be confused 

 by their recital. In connection with the following statement, the 

 accompanying diagrams (figs. 459-461) will assist in indicating the 

 historical connections of the groups, and also to some extent the rela- 

 tive periods of time involved. 



The paleozoic groups Cycadofilicales and Cordaitales represent 

 the historical background of gymnosperms. They are of equal age, 



