THE QUEST OF PHYLETIC LINES 109 



said that the Leptosporangiate Ferns are of all Vascular Plants 

 those to which the canons of phyletic treatment may be most 

 hopefully applied. Their origin is not entirely lost in the anti- 

 quity of their story. They present a variety of characters of exter- 

 nal form and habit, of anatomy, of sorus and sporangium, of 

 prothallus and sexual organs, which can be traced, and their paral- 

 lelism compared. The results can be checked by reference to 

 fossils of successive horizons, and lastly, since the several types 

 are for the most part adequately represented by living species, 

 the check of physiological probability can be applied, and even 

 experimentally tested. Though the genetic relations appear 

 here to be as involved as in any well represented division of 

 plants, still in combing those relations out into true phyletic 

 sequences the work may be conducted with as high hopes, and as 

 truly scientific a foundation as in any section of the vegetable 

 kingdom. This is still far from being accomplished. But if it 

 be pursued in a broad scientific spirit, the result will probably 

 serve as an object lesson, which may be applied in laying down the 

 canons of comparison for other and still more difficult series of 

 plants. 



