Common, French, and Burbank Marigolds
The large double Burbank marigolds have been developed from the small single flowers through selective breeding. By cross-fertilizing different varieties, a tendency to variation is induced, and by preserving for seed purposes only those flowers which vary in the desired direction, any given quality is accentuated, until finally such a metamorphosis as this is brought about. The rapidity of development varies with different plants, but many generations are required to produce such a transformation as that shown in this picture.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 1