352 Causes and Course of Organic Evolution 



wliile the more humble types that constitute the Equiseteae 

 or true horsetails began to evolve, possibly as a sideshoot from 

 herbaceous carboniferous representatives, and this from the 

 Jurassic period to the present day has alone survived, as rep- 

 resenting the once abundant group of palaeozoic age. 



But coeval "^^'ith them in origin seem to have been three 

 great group-series, that for convenience we may designate 

 respectively the Archseocordaiteae, the Archseofilicinese, and 

 the Archieocycadese, which were already established during 

 the upper devonian, and whose earliest evolution from primi- 

 tive cellular thalloid forms must have dated back to the Cam- 

 brian period at least. 



The material is still entirely lacking that would enable us 

 to follow or reconstruct the early pathway taken by these in 

 branching off from a common ancestry. But, piecing together 

 such details as living examples and fossil specimens furnish, 

 we would consider that such lines of advance as are indicated 

 in the diagram facing p. 356 (Fig. 13) were pursued. 



All three, from an originally common kind of green cellular 

 thalloid expanse or prothallus, seem to have produced, from 

 the fertilized egg borne on it, an alternating sporophyte gen- 

 eration that gave rise to successive stem and leaf parts, that 

 were joined to or reared on each other as several phyta that 

 made up a vegetative system. In the floating aquatic form 

 of Ceratopteris we get some notion of what such a generalized 

 type may have been. Here on short successive stem seg- 

 ments simple floating thalloid leaves are formed, that are 

 traversed by delicate vascular bimdles. Along the edges of 

 the leaves abundant buds are produced that separate and form 

 new plants. But equally might swellings have formed on 

 these that could have matured into sporangial sacs, and so 

 probably arose that still aquatic group — the Hydropteridese — ■ 

 that in the long-dra^Mi ages since has branched out into four 

 distinct representative genera still living. 



But, with increasing develo])ment of the sporo])hyte stem 

 and roots, with increasing }issumi)tion of a land existence by 

 the sporophyte, and with evolved tissue elements that were 



