REVIEW AND SUMMARY. 243 



thirty orders of dicotyledons, including about one hundred 

 and sixty-three families, and seven orders of monocotyle- 

 dons with about forty families, while the gymnosperms 

 include three orders with thirteen families. 1 The orders 

 themselves are associated in higher groups, which in their 

 turn make up the great classes just named. 2 



Another fact of prime importance, that cannot well have 

 escaped the student's attention, is the gradually increasing 

 complexity of structure, particularly of the floral Progressive 

 organs, met with as we proceed from more prim- ^fl^f 10 

 itive to more advanced families. Comparing a organs, 

 lily, for example, with an orchid, or a buttercup with a 

 dandelion, it is plain that the flowers of the higher families 

 have undergone very remarkable changes of form and 

 structure, although the fundamental plan may still be 

 recognized. These changes of structure represent, as a 

 rule, .progressive adaptation to cross-fertilization through 

 the agency of insects. It appears, too, from all we can 

 learn of them by comparative study, that these progressive 

 modifications have taken place step by step with corres- 

 ponding modifications of structure and habit on the part 

 of their visitors. The history of such a flower as that of 

 the sweet-pea or violet, of the milkweed or daisy, must, if 

 this view is correct, reach far back into the past, so far that 

 the imagination fails to reproduce the long series of changes 

 that have taken place in the succession of intervening 

 generations. A glimpse of this history, helpful and satis- 



1 Cf. Luerssen, Botanik, Bd. 2, pp. vii-x. 



2 These groups of a higher order are less satisfactorily defined. For an 

 attempt at their systematic presentation, see Goebel, Outlines of Classifi- 

 cation and Special Morphology, pp. xi, xii. The student will do well to 

 remember that all such attempts to represent the affinities of families and 

 higher groups involve more or less uncertainty, and that all classifications 

 are of necessity provisional. 



