72 THE THEORY OF EVOLUTION 



uniform stamp, which can easily induce the idea of 

 close relationship and in some cases has even led to the 

 putting forward of mixed types or transitional forms. 

 Now, however, we can impute many peculiarities 

 of the great groups of the Carboniferous flora to con- 

 vergence without conflicting with the facts observed. 

 The increase of thickness of the stem by means of a 

 constantly present embryonal tissue (Cambrian) in 

 many of the Lepidophytes and the Calamariacese, 

 the possession of large subterranean rhizomes * (Stig- 

 maria), 1 the club-like thickening of the stem below, 

 the possession of a smooth trunk without bark, the 

 formation of stomata on the stem or in organs proper 

 (Pneumatophora), can nay, must be regarded as 

 adaptive factors to the like environmental conditions. 

 The proof of this has been provided by Potonie in his 

 work already mentioned ~ in the most convincing manner 

 by the comparative study of the Carboniferous flora 

 and that of an existing tropical marshland. Indepen- 

 dently of the systematic differences between the fossil 

 and the present moor flora in the Carboniferous era 

 Pteridophytes and Gymnosperms and, in the equivalent 

 tropical moorland of Sumatra, Dycotyledons the 

 peculiarities of both flora are unequivocally explained 

 by the mode of life under the same conditions. 



1 Rhizomes (root-stocks) are extensions or thickenings of the under- 

 ground stem, from which the roots proper issue. The ' stigmaria ' are very 

 widely projecting twice-branched formations, whose surface is covered 

 with round scars (whence the name stigmaria) spirally arranged, which 

 stand far apart from each other, and sometimes bear round rootlets. 

 E. Frans : Der Petrefactensammler, Stuttgart, 1910, p. 48. 



2 Die Entstehung der Steinkohle, u. s. w., pp. 152, 166, and 169 (tables). 



