146 Plant Life and Evolution 



trees. The origin of the conifers is uncertain, but 

 there is some reason to assume a descent from 

 forms allied to the giant club-mosses of the Coal 

 Measures. The affinities of the other order of gym- 

 nosperms, the Gnetales, are not at all clearly estab- 

 lished, nor is it certain that the three known 

 genera are related among themselves. There is 

 some ground for an assumption of a relationship 

 between some of these and the lower angiosperms ; 

 but the origin of the latter, the predominant modern 

 type of plants, is very far from clear. 



The Living Seed-plants Not All Related. It is, 

 however, extremely likely that the existing seed- 

 bearing plants do not form a homogeneous class, 

 but more properly should be considered as a class 

 representing several quite independent develop- 

 mental lines, some of which may not be related at 

 all. The lower types, or gymnosperms, show evi- 

 dent relationship with the pteridophytes, but the 

 different orders may very well have been derived 

 from quite different pteridophytic stocks. The 

 origin of the modern type of flowering plants, the 

 angiosperms, is exceedingly obscure. One group 

 of seed-plants, the Cordaitales, the earliest known 

 type, became extinct at the end of the Paleozoic, and 

 probably has left no descendants. 



