356 Causes and Course of Organic Evolution 



cotyledons, are all characters in common that distinguish 

 the six groups from other phyla. 



Did space permit, detailed reference might be made to the 

 structure of the elements that constitute the stem of all of 

 the above. It T\dll suffice now to say that alike in the cellular, 

 the sieve, the tracheidal, and other elements a surprising simi- 

 larity exists, alongside certain important differences in group- 

 ing, that constitute family distinctions. 



Reference to the phylogenetic diagram (Fig. 13) will set 

 before the reader the ramification of another, and according 

 to the writer the specially dominant, line of evolutionary 

 advance, that started from a nearly related source with the 

 two great polyciliate lines already considered. This proto- 

 cordaital stirp has had an equally ancient origin with the other 

 two. For in devonian rocks of America, and in the lower 

 carboniferous or "calciferous" rocks of Scotland, abundant 

 remains of the leaves and tTvigs, or at times giant trunks which 

 floated some distance before being fossilized, occur. 



When we consider their extremely high organization, and 

 attempt to correlate this with form-evolution in connection 

 with time-duration, we are compelled to consider that they 

 had already developed as primitive vascular plants during 

 the Silurian age. But in this case, as for the two great phyla 

 already studied, the softer and more ancient remains that 

 might have guided us as to their primitive origins are entirely 

 unknown. Again, therefore, we must endeavor to piece to- 

 gether information derived by comparison of fossil and living 

 examples. 



As compared with the equisetineous, the archaeofilicineous, 

 and the archfeocycadineous groups, the archseocordaital ances- 

 tors must very early have matured the stem segment of the em- 

 bryo sporophyte as the prominent and steadily-growing mem- 

 ber, at least in some genera. The leaves on the other hand re- 

 mained simple or only lobed, were usually lanceolate, and had 

 a few to many parallel or sometimes branching veins. The 

 primitive monostele of the evolving vascular sporophyte stem 

 must very early have divided into a set of four or more meri- 

 steles, that were placed in a circle round a quantity of cellular 

 pith tissue. 



