INTROGRESSION AND EVOLUTION 71 



changed morphologically, it is now able to colonize a much 

 larger territory than that to which it had previously been 

 restricted, and it does, in fact, become almost ''weedy" in 

 its habits. 



Meanwhile, by other routes, man has unwittingly carried 

 his new weed P. sativa into the area of P. occidentalis. There 

 the two hybridize and the hybrids backcross to P. sativa, in- 

 creasing its variability still more. From the resulting inter- 

 mixture there is bred a new and particularly aggressive form 

 of this weed which spreads around the world and eventually 

 becomes recognized as P. sativa var. peregrina. 



So much for a part of the history of domestication in the 

 hypothetical genus Planta. Let us now consider the diffi- 

 culties of unraveling this history had Planta been an actual 

 genus. We would have had little or no evidence about it as 

 it occurred in prehuman or even in early human times. From 

 the bewildering array of specimens in our herbaria, collected 

 by different people and in a more or less haphazard fashion, 

 from notes by agronomists who had cultivated P. utilis, and 

 from our own powers of observation we should have had to 

 put the story together. This would have been difficult. 

 Someone interested in P. sativa might never have been able 

 to make field studies in the original region where intro- 

 gression took place so actively in P. mixta. Only occasionally 

 would careful local field studies reveal to the scientific world 

 such interesting phenomena as the effect of P. utilis on P. 

 endemica. Were the work to be done by purely conventional 

 taxonomic methods, based upon the critical study and com- 

 parison of single specimens, a first-rate taxonomist might 

 separate the genus into the following categories: (1) endem- 

 ica, (2) mixta, (3) utilis-sativa, and (4) occidentale. From 

 collections of single individuals it would not be possible to 

 distinguish between the original endemica and its variety 

 robusta. One could not in every instance separate some var- 

 iants of sativa from some of those of utilis. Planta sativa 

 peregrina could not be differentiated from sativa, and the 

 intergrades between punctata and cruciformis would be con- 



