3G0 PRACTICAL COURSE IN BOTANY 



shows many windings and deviations from the direct route. 

 The monocotyls furnish a conspicuous example of this de- 

 parture from the general law of progression. It was formerly 

 supposed, on account of their greater simplicity of structure, 

 that they were a more ancient type than dicotyls, but recent 

 investigations point to the conclusion that they are a later 

 offshoot, derived from some primitive form of aquatic dicotyl, 

 and represent, not an ancient aT5d primitive stock, but a case 

 of retrogressive evolution from a higher type. Strong pre- 

 sumptions in favor of this view are : (1) that various species 

 of dicotyls show an unequal development of the seed leaves, 

 amounting, in the bryony, to complete abortion of one of 

 them, while some monocotyl seeds show morphological 

 characters that can best be explained as survivals, or inherit- 

 ances, from a dicotyl ancestor; (2) the structural resem- 

 blances between gymnosperms and dicotyls are closer than 

 between gymnosperms and monocotyls, which could hardly 

 be the case if the latter were the more ancient ; (3) the geo- 

 logical record does not show them to have appeared before 

 dicotyls ; (4) the number of cotyledons furnishes no criterion 

 as to the relative age of any plant group, since all three types 

 are represented among the pteridophytes, where plants are 

 found bearing one, two, or more cotyledons. 



The theory of their comparatively recent origin from an 

 aquatic ancestor is further borne out by the many points of 

 similarity between their internal structure and that of hy- 

 drophytes (318), and also by the great proportion of aquatio 

 plants among them, amounting to thirty- three per cent, while 

 in dicotyls the proportion is only four per cent. Can you 

 give any reasons, from your examination of their internal 

 structure (113, 114), for believing that the line of develop- 

 ment which they have followed is a less effective one for 

 meeting conditions now existing on the globe than that at- 

 tained by dicotyls ? 



We should remember, too, that while progressive evolution 

 implies successful adjustment to surroundings, it is possible 



