CHAPTER XXXVI 
THE EVOLUTION OF PLANTS 
489. The Problem Stated. If we knew the entire 
history of development of the plant world, we could ar- 
range all plants now living, and that have lived, so as to 
show their genetic relation to each other. The prob- 
lem is illustrated on a small scale by various related culti- 
vated plants, all known to be derived from a common 
wild ancestor. Cabbage and its relatives are a case in 
point. The botanical relatives of the cabbage include 
such forms as kohlrabi, brussels-sprouts, collards, kale, 
broccoli, and cauliflower (Fig. 397). All of these garden 
vegetables are believed to have been derived from the 
common wild cliff-cabbage (Brassica oleracea) of Europe 
and Asia, by selecting mutations in various directions, 
e.g., excessive development of the stem in kohlrabi, of 
the terminal bud in cabbages, of the lateral buds in brus- 
sel's sprouts, of the flower buds in cauliflower. Or, to 
refer to de Vries's studies in experimental evolution, where 
the course of descent was actually observed, we may 
arrange the forms of Lamarck's evening-primrose so as 
to show their known derivation. 
The general problem, therefore, is to establish the 
genetic relationship of all known plants, living and fossil. 
Since the Angiosperms stand at the top of the series, the 
problem resolves itself largely into ascertaining the 
phytogeny, or line of ancestry, of that group. 
569 
