THE ORIGIN OF THE VASCULAR PLANTS. 281 



difficulty seemed insurmountable, and more recently another 

 theory was advanced, of which Goebel is the principal 

 exponent at present. This theory holds that the origin of 

 the Pteridophytes must be sought very low down in the 

 Archegoniate series before they had assumed the characters 

 of the modern Muscineae, and that the two series, Muscineae 

 and Pteridophyta, as they exist at present, form two entirely 

 divergent lines of development arising from common 

 ancestors, that in structure were intermediate between the 

 green Algae and the simplest Muscineae. This theory is 

 mainly based upon the alga-like prothallium of certain 

 species of Trichomanes, a genus of the Hymenophyllaceae. 

 The latter are Ferns, mostly of small size and extreme 

 delicacy, mainly tropical in their distribution. In the smaller 

 species the very delicate filmy leaves remind one strongly 

 of the leaves of some of the larger Mosses, such as Milium, 

 and this resemblance led the older botanists to suggest a 

 possible relationship between them, and later investigators 

 have also been led by the apparent simplicity of their 

 structure to assume that the Hymenophyllaceae are the 

 most primitive of existing vascular plants. 



Goebel pointed out the resemblance between the 

 prothallium of Trichomanes and the filamentous protonema 

 of most Mosses, and from a study of the former, as well as 

 that of some of the true Mosses, especially the peculiar 

 genus Buxbaumia, came to the conclusion that the primitive 

 form from which the other Archegoniatae have sprung was 

 a branching alga-like filamentous form, bearing the sexual 

 organs directly upon the filament, and that the flat thallus 

 of the Liverworts and most Fern prothallia is a secondary 

 development, the two having nothing in common. Un- 

 fortunately Goebel does not offer any explanation of the 

 origin of the sporophyte of the Hymenophyllaceae either 

 from forms such as exist at present or from hypothetical 

 ones, but confines his attention almost exclusively to the 

 sexual generation. This one-sided view of the question 

 weakens very much the force of his arguments, and serious 

 objections to his theory arise as soon as the subject is 



considered from other directions. Even putting aside the 



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