434 GYMNOSPERMS 



and the sperms of Taxus do not seem to be very far removed from a 

 swimming ancestry. Ginkgo, like the cycads, has retained the swim- 

 ming sperms, after other structures have advanced. Nevertheless, 

 the swimming sperm is a universal pteridophyte character, which 

 Ginkgo has inherited directly or indirectly from the pteridophytes. 



On the whole, it seems best to derive Ginkgoales from the Cor- 

 daitales. Some of the seeds assigned to Cordaitales bear a close re- 

 semblance to those of the living Ginkgo, especially in the archegonia 

 and "tent pole" structure of the top of the female gametophyte. 

 Material recently secured, and now under investigation, may throw 

 some light on resemblances between Cordaites and Ginkgo. The 

 seeds of the Cordaitales are already so advanced that they are far 

 removed from the primitive seeds which must have existed while the 

 lower coniferophytes were emerging from their heterosporous pteri- 

 (i()j)hyte ancestry. 



The Coniferales have so many cordaitean characters that they 

 must have come from the Cordaitales, or the two groups must have 

 developed independently from a common ancestor. In this case, 

 it hardly seems necessary to call upon parallel development, as is 

 generally done when there is difficulty in making a direct connection. 



The Coniferales fall, rather naturally, into two great groups, the 

 Pinares and Taxares, the former containing four families, Abietaceae, 

 Taxodiaceae, Cupressaceae, and Araucariaceae; and the latter two 

 families, the Podocarpaceae and Taxaceae. 



Most of the literature dealing with the phylogeny of gymno- 

 sperms is concerned with the origin and relationship of these six 

 families, and it is here that the most intensive investigations have 

 been made. Life-histories in these six families are so well known that 

 theories are based upon interpretation of a wide range of established 

 facts. 



More investigations and more theorizing have been devoted to 

 the Abietaceae and Araucariaceae than to all the other four together. 

 Which of these two families is more ancient? Did both arise inde- 

 pendently from the Cordaitales or some other ancestor, or did one 

 of them give rise to the other? These are questions which are not yet 

 settled. 



Most of the fossil material consists of stems, roots, and leaves, 



