The Origin of Species 309 



no doubt that such determinate variation is a com- 

 mon phenomenon among plants. Among the most 

 striking of these cases are the several quite unrelated 

 cases of sex-evolution in the lower plants, as well 

 as its development in animals, and also the remarka- 

 ble resemblances already referred to in the structure 

 of the nuclei and the complex details of nuclear 

 divisions in plants and animals. The striking simi- 

 larities in the evolution of the flowers in mono- 

 cotyledons and dicotyledons are also an excellent 

 example of such determinate variation. The devel- 

 opment of sympetalous flowers has occurred a num- 

 ber of times in both monocotyledons and dicoty- 

 ledons, and the same is true of the inferior ovary, 

 and the development of zygomorphic flowers, which 

 characterize the most specialized types in both 

 classes. These have been developed quite inde- 

 pendently in response to the same needs, in this case 

 pollination by insects and birds. The development 

 of special colors in flowers as a means of attracting 

 birds and insects, e.g., the frequent occurrence of 

 bright scarlet in ornithophilous flowers, and the de- 

 velopment of nectar and scent, may very well be 

 cited as examples of such definite or determinate 

 variation. Many other instances might be cited, but 

 one more will suffice. The character of the leaves 

 of whole families or genera is often as character- 

 istic as the flower. In the vetches we expect to 

 find the tendril always assuming the place of the 

 terminal leaflet, while in Smilax quite as uniformly 



