144 T O R R E Y yV 



to year in the region where the crop is indigenous, and that the variabiHty from 

 year to year increases with departure from that center.' In the first place, note 

 that Hyde indicates that the crop production is not necessarily high at the 

 region of center, or where the crop is indigenous, but only that it is uniform 

 from year to year. This does not fit well with criteria two and four. Further- 

 more, it does not appear that the term "indigenous" is employed in its strict 

 meaning of being "native," but in a more general meaning of being "at home" 

 in the sense of being well adapted. It is, of course, well known that crop produc- 

 tion shows the greatest stability from year to year in climatic areas to which it is 

 best adapted. This phenomenon appears to have nothing to do with center of 

 origin of the crop (Vavilov, 1928, 1940), but is explained by weather and the 

 operation of limiting factors (Taylor, 1934). 



Criterion 6. Continuity and Convergence of Lines of Dispersal 



AMien the species of a genus or higher category are distributed along natural 

 highways of migration, and when these highways converge on a certain area, 

 the distributional pattern suggests that the region of convergence of these 

 routes is the center of origin and dispersal. This suggestion is even stronger 

 when, as is usually the case, unrelated organisms show the same pattern. There 

 is, however, no a priori reason considering dispersal lines alone why migra- 

 tions need have been divergent from the apparent center rather than convergent 

 on it. It is usually not difficult, however, to obtain evidence (see criterion eight) 

 as to which direction the migrations took. Such evidence is largely obtained 

 from comparative morphology and relationships. Sometimes paleontological 

 evidence helps indicate the direction of migration. In other cases cytogenetical 

 analysis of the related forms reveals without doubt the direction which the 

 movement has taken. ]\Iigratory tracts are merely lines (however broad) of 

 frequent, suitable habitats, and are not necessarily one-way routes. As ex- 

 pressed, and by itself, the criterion is not valid. 



Criterion 7. Location of Least Dependence upon a Restricted Habitat 



The use of this criterion for the indication of center of origin depends upon 

 a species being more polymorphic at the center of origin ( Criterion 1 ) or upon 

 more primitive forms having wider tolerances than derived forms. Both of 

 these conditions may not be true. A wide species contains a very large number 

 of biotypes, perhaps many thousands (Turesson, 1925, 1932; DuRietz, 1930). 

 Progressively from the center of origin, and especially along narrow migratory 

 tracts extending from the main area, there is a biotype depauperization. This 

 can result from partial isolation due to distance alone. A remote portion of a 

 population does not in practice, even if in theory, have access to the entire 



-Recent investigations are summarized by Klages (1942). 



