372 Causes and Course of Organic Evolution 



symmetrj\ But until more exhaustive and extensive studies 

 have been made from the ecological, the experimental, and 

 the taxonomic standpoints, as to foliar and floral variations 

 in symmetry, it seems superfluous to advance any explanation. 



From all the morphological evidence now to hand, we would 

 consider that monocotyledons branched off, probably during 

 early Jurassic times, from a parent stock that showed pro- 

 nounced dicotylar, and to a less degree gnetal structural de- 

 tails in root, stem, and seed-leaves, but occasional incipient 

 monocotylar details in foliage-leaves. Largely in line \\ith 

 Henslow's position, we would further consider that the group 

 evolved through exposure to moist or in some cases aquatic 

 environment and shade conditions. But such probably alter- 

 nated in many cases "uith exposure to hot dry atmospheric 

 states. The former may thus have evolved the geophilous 

 and soft-foliage types of monocotyledon, the latter the semi- 

 xerophilous or xerophilous types. 



All morphological and pal^ontological evidence however 

 would lead us to consider that the monocotyledonous stock 

 did not arise as a monophyletic, but at least as a diphyletic 

 series. For the presence in older cretaceous rocks of pandani 

 and palms, and in these rocks or in late cretaceous rocks of 

 gramineous, araceous, and other herbaceous types, would 

 indicate at least two, if not three to four, important though 

 diverging lines of evolution. But here and now it would be 

 inappropriate to go further. 



The main line of advance that culminated in the dicotyledons 

 of existing world floras must have been fully differentiated 

 from monocotyledons, and must have subdivided into minor 

 lines of evolution during later Jurassic times. For in this 

 way only can we explain the varied remains that have been 

 met with in early cretaceous rocks of America and Euroi)e. 

 But we would consider that the remains now known represent 

 a mere fraction of the genera and particularly herbaceous ones 

 that had then appeared. For if we attempt to group to- 

 gether the few localities of present-day age, or of pleistocene 

 date, where considerable plant remains have been found fossil- 

 ized, the localities are few indeed, and the remains are scant 



