68 INTROGRESSIVE HYBRIDIZATION 



largely man-made and inhabit ecological niches that are 

 either directly or indirectly the results of man's interference, 

 our ^'explanation" of the origin of such a crop is merely the 

 posing of a much larger problem. Where and how were the 

 cultivated plant and its related weeds bred out of the pre- 

 human elements in the genus? Most of our cultivated plants, 

 therefore, merely tell us that evolution has proceeded apace 

 under domestication. Few of them are the kind of research 

 material from which we can get a precise answer as to how 

 the changes that occurred under domestication were brought 

 about. 



Accordingly, we shall first present (in simplified, pictorial- 

 ized form) a hypothetical, generalized diagram of the way in 

 which domestication of weeds and cultivated plants most 

 probably took place. With that for reference, there will 

 then be presented detailed evidence from various genera sup- 

 porting the hypothesis. Plate 3, therefore, is a diagram of the 

 way in which cultivated plants and weeds have been con- 

 sciously and unconsciously developed from their wild pro- 

 genitors. It is greatly simplified as compared with the actual 

 history of most cultivated plants and weeds. For one thing, 

 the special and complicating effects of polyploidy and 

 apomixis are not included. With the occurrence of apomixis 

 or of ploidy either before or after domestication, further 

 complications would be added to the existing complexities 

 of relationship. 



Turning to Plate 3, the diagram at the top of the plate con- 

 cerns the five original entities in our mythical genus Planta 

 and their fate under the influence of man. The diagram 

 represents an area of continental size with one highly local- 

 ized species, "P. endemica/^ in the east, and another species, 

 ''P. occidentale/^ in the far west. In the center of the con- 

 tinent are three entities, "P. laxa^^ and the two entities that 

 we have grouped under "P. mixta,^' the variety ^'cruciformAs'^ 

 and the variety "punctata.^' Planta cruciformis and P. 

 punctata are fairly v/ell differentiated and for the most part 

 occupy different areas, but in the zone where they approach 

 each other (even in prehuman times) there was some hy- 



