CHAPTER IX 

 EVOLUTIONARY TENDENCIES AMONG GYMNOSPERJ^IS 



The gymnosperms have been presented group by group, but they 

 should be viewed also as a whole. It would be possible to summarize 

 the facts in reference to each structure, but this not only would involve 

 needless repetition, but also would fail to distinguish between the 

 facts that are significant and those that are relatively insignificant. 

 There is no better way in which to develop a clear picture of a great 

 group than to select those facts of structure that enter into its general 

 evolutionary history. This has nothing to do with differences among 

 species, or even among genera, which may be left to the taxonomist; 

 but it deals with those general tendencies to change structures which 

 can be noted in passing from the most ancient gymnosperms to the 

 most recent. 



While the discovery of these tendencies aids in reaching conclu- 

 sions in reference to the phylogenetic connections of the groups of 

 gymnosperms, it must be remembered that the tendencies are facts 

 and the phylogenetic conclusions are very uncertain inferences. 

 Moreover, a general tendency expresses itself throughout a great 

 group, and has to do with the transition from ancient to modern 

 forms, rather than with the breaking up of the group into several 

 phylogenetic lines. Failure to remember this fact has been respon- 

 sible for much sterile inference as to relationships, similar stages in 

 some general tendency being assumed to mean immediate genetic 

 connection. The organism is a plexus of structures, and must be 

 considered in its totality when relationships are being considered. 

 Among the general tendencies leading to the origin of seed plants, 

 that which resulted in heterospory must be regarded as of paramount 

 importance; and yet it is clear that heterospory arose several times, 

 and probably many times, independently, as the natural result of a 

 general tendency among pteridophytes. To put into the same genetic 

 group all heterosporous pteridophytes would be regarded now as a 

 morphological absurdity; and yet there have been repeated sugges- 



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