582 THE AMERICAN NATURALIST [Vol. LI 



characters in higher plants have originated in this way. 

 Such morphological generic characters are found in num- 

 bers wherever one turns. They indicate a great variety 

 of marked changes, in addition to those involving altera- 

 tion in number of parts; they most often concern the 

 flower structure, on which generic differences usually de- 

 pend; and in many cases at least they can not reasonably 

 be supposed to be of any special value to the plant. 



If we turn to the lily family and compare the well- 

 known North American genera, Smilacina and Maianthe- 

 mum, we find the following differences : 



Smilacina Maianthemum 



These generic differences are almost entirely in the 

 number of parts in the flower. Otherwise, in foliage and 

 habit Maianthemum might be considered a reduced boreal 

 subgenus of Smilacina. Can it be doubted that Maian- 

 themum originated from the ancestors of certain species 

 of Smilacina through a mutation, in which the flowers 

 changed suddenly from the hexamerous to the tetramerous 

 condition? All these changes in flower-parts would then 

 have occurred at one stroke. It can not well be imagined 

 that they passed through a series of gradual transition 

 stages which have since been lost. When one remembers 

 the almost universal occurrence of 3-parted flowers in 

 Monocotyledons, this change becomes all the more strik- 

 ing. The whole order Cruciferae, among Dicotyledons, 

 must have originated in the same way, through a sudden 

 change from pentamery to tetramery. 



If we examine the species of Maianthemum and their 

 varieties we find evidence that similar processes of dis- 

 continuous variation are going on at the present time. 

 The genus contains three species, M, canadense in Amer- 



