XLV] COMPARISON 231 



selves, even if no other Ferns were in existence. There is reason to believe 

 that the Dipteroids have come down through the ages following their own 

 evolutionary sequence as a clear line. That the progressions which they 

 show are so nearly parallel with those seen in other phyla, though these 

 appear to have been distinct from them since Mesozoic time, affords one 

 of the most impressive aspects of their study. And yet in this the 

 Dipteroids do not stand alone. They are, however, among the most 

 striking witnesses to the far-reaching effect of homoplastic development. 

 If any one feature were called for that should stamp the independent 

 character of this phylum, it would be wise to point out the amplification of 

 the sorus in relation to the sub-soral vascular system ; for this appears to be 

 a method of advance peculiarly its own. 



The palaeontological and geographical history of the Matonioid-Dipteroid 

 phylum of Ferns has been discussed with special knowledge by Seward in 

 his Hooker Lecture {Linn. Jonrn. 1922, p. 227). Notwithstanding their 

 present localisation in the Malayan region these Ferns appear to have been 

 cosmopolitan in their spread in former days (compare Map B, Vol. II, 

 p. 329). Their geological record clearly establishes the fact that certain 

 genera were formerly inhabitants of many parts of the world in which they 

 are now unknown, and their present restricted distribution is best interpreted 

 as evidence of declining vigour, or as an expression of inability to hold 

 their own in competition with more recent products of evolution. The 

 problem of the original home of the Matonioid-Dipteroid stock is not easy 

 of solution. When the fossil forms first appear among the records of the 

 rocks, certain genera had already reached a vigorous stage of development 

 in Europe and North America: by the Rhaetic period they were thoroughly 

 established in the Tonkin region, also in Germany, Greenland, and Scania. 

 This wide distribution at an early time will aid the interpretation of some of 

 the suggested derivatives of the present day. It would appear possible that 

 while these are represented in the Malayan region by such genera as 

 Cheirople7iria, Christopteris, Hymenolepis and Neocheiropteris, and ultimately 

 by Pleopeltis with its eastern preponderance, Platycerium had had a wider 

 spread from early times. On the other hand, the Polypodioid stock PJilebodium 

 may represent Ferns of similar origin which progressed in the American 

 region, along lines parallel to but distinct from their preponderantly eastern 

 relatives of the genus Pleopeltis. 



Here, however, we approach problems which lie beyond the scope of the 

 present work. The pursuit of them into phyletic and systematic detail must 

 be left in the hands of those who come after, armed with special knowledge 

 and provided, perhaps, with fresh criteria of comparison. 



