[CH. 



166 CONIFERALES 



recognised as embracing several recent genera, into genera implying 

 a limitation of affinity within narrower bounds has, I venture to 

 think, been carried too far. The investigation of fossil coniferous 

 wood, in spite of the disappointing quality of the data from the 

 systematist's point of view is well worth attention. An examina- 

 tion of fossil wood from different geological horizons brings to 

 light many striking instances of a mixture in single plants of 

 features now characteristic of distinct genera. It is the generalised 

 forms that throw light on the nature of the changes produced in 

 anatomical structure in the course of evolution. The older and 

 more generalised types are of special importance to the student of 

 phylogeny. The very difficult question as to the stock from which 

 the Conifers are derived is too wide to be adequately discussed in 

 a general treatise. It is probable that the Coniferales are mono- 

 phyletic, the Araucarineae being the oldest representatives of the 

 group while the Podocarpineae are a closely related series. The 

 widely held view that the Araucarineae are descended from Cordai- 

 talean ancestors is by no means definitely established; it rests 

 mainly on anatomical evidence and the arguments based on a 

 comparison of the reproductive shoots are far from convincing. 

 On the other hand those who favour a Lycopodiaceous ancestry 

 for the Coniferales are confronted with difficulties which, though 

 I venture to think they are not insurmountable, have not been 

 adequately met 1 . The suggested linking up of the Cordaitales, 

 through types in which the cylinder of secondary xylem is sup- 

 plemented by separate primary strands of vascular tissue, with 

 Lyginopteris and other Pteridosperms leads to the inclusion of the 

 Coniferales among the descendants of an ancient Filicinean stock, 

 but here too the chain of evidence is incomplete particularly as 

 regards the lack of data as to the nature of the reproductive organs 

 of several Palaeozoic genera founded on anatomical characters. 



The problem is still unsolved: the discovery of additional 

 types and a more thorough comparative study of such data as we 

 possess may enable us to see more clearly the paths along which 

 evolutionary tendencies have operated, but the absence of records 

 of the vegetation of pre-Devonian times deprives us of the means 

 of following to their common source the different phyla of vascular 



1 For a useful summary of arguments see Burlingame (15). 



