INTROGRESSION AND EVOLUTION 



69 



p. faxa 



P. occidentale 



P. mixta 



var. cruciformis 



P. mixta 

 var. punctata 



P. endemica 



P. sativa 



P. utilis 



P. sativa 

 var. peregrina 



P. endemica 

 var. robusta 



Plate 3. Introgression under the influence of man. Diagram showdng 

 the role of introgression in building up cultivated plants and weeds in the 

 hypothetical genus Planta. The ranges of the various species and varie- 

 ties are represented upon an area of supposedly continental size. The 

 plate shows the ranges of the species and varieties in prehuman times (at 

 the top), then the successive steps by which the present condition (bot- 

 tom of the plate) has been brought about. Further discussion in the 



text. 



