132 EDGAR ANDERSON AND WILLIAM L. BROWN 



phenomena in advance of the published facts, and the existence of these 

 strong linkages has been more apparent to some than to others. 



WIDTH OF CROSS 



The demonstration that Corn Belt dents largely are derived from hy- 

 bridization between Southern Dents and Northern Flints is of particular im- 

 portance because this is such a wide cross. Our evidence for this assertion is 

 largely morphological, though there is supporting evidence from cytology 

 and genetics. 



In nearly all species of cultivated plants there are conspicuous differences 

 in color and shape. These differences give the various cultivated varieties of 

 a species a false aspect of difference from one another, and from their wild 

 progenitors. False, because these differences are usually due to a few genes, if 

 not being actually monofactorial. The striking differences between such 

 varieties are therefore no true indication of the distinctness of their germ 

 plasms. 



On the other hand, there are subtle differences in form, proportion, and 

 indument which, though difficult for a novice to apprehend, are more like 

 the differences which distinguish distinct species of the same genus. These 

 taxonomically important differences have proven valid criteria for indicating 

 the diversity of germ plasms. So it has been proven that the subtle taxonomic 

 differences between the Old World and New World cottons are much more 

 representative of the genetic diversity and relationships of these two groups 

 of varieties than are the conspicuous differences in color and leaf-shape which 

 are found within each group. In the Cucurbits the striking differences in 

 color and form of fruit, which differentiate the varieties of Cucurbita Pepo 

 and of C. moschata, are superficial compared to the taxonomically significant 

 features which separate these two groups. The latter, moreover, have been 

 proved to be a significant index of genetic diversity, either between these 

 two groups of Cucurbits or in assaying the variation within C. Pepo itself 

 (Shifriss, 1947) (Whitaker and Bohn, 1950). 



The difficulty in relying upon such taxonomic criteria is that the method 

 is highly subjective. Taxonomy is of necessity still more of an art than a 

 science. This means that one must personally examine the evidence if his 

 opinion is to be worth anything. It also means that the worker's opinion is 

 worth no more than his understanding of the taxonomic entities included in 

 his judgment. However, until more objective criteria are evolved for this 

 field, we shall have to use fairly traditional taxonomic methods for want of 

 anything better. Accordingly, the senior author has for two years spent one 

 day a week in a technical, agrostological, herbarium survey of all the grasses 

 conceivably related to Zea mays — all the genera in the tribes Andropogoneae 

 and Maydeae. With that background, his judgments may well be mistaken 

 but they are certainly informed. 



From this point of view, the variation within Zea mays is without parallel, 



