MAIZE— KEMPTON 403 



In maize, mutations are almost the rule, the germ plasm being in a 

 highly unstable condition as compared with that of its wild relatives. 

 Within the past 20 years over 300 heritable variations or mutations 

 have been discovered and the nature of their inheritance determined. 

 Some of these affect the structure and appearance of the entire 

 plant, and if considered strictly from a systematic botanical point 

 of view, plants possessing certain ones of these mutated genes could 

 not be classed as maize but would fall more naturally into the sorghum 

 tribe. Indeed the mutation loiown as teopod, the result of a change 

 or mutation in a single gene, is of sufficient magnitude to give to the 

 origin of maize by mutation strong superficial support in that it 

 illustrates what profound modifications can occur as the result of a 

 single gene change (pi. 27). 



The most serious objection to the hypothesis that maize originated 

 from teosinte by mutation is found in a study of the hybrids between 

 these two genera. The characters that differentiate the two genera 

 do not behave in inheritance like mutations. The ear of corn does 

 not segregate as a unit or a few units from teosinte-corn crosses 

 and so with the other characters (pis. 28 and 29). These hybrids 

 show clearly that the two genera differ by hundreds if not thousands 

 of genes or on the mutation idea that corn differs from teosinte not 

 by a few major mutations but by a multitude of small ones. This 

 fact reduces the mutation theory of origin to that of the selection 

 of small variations. In reaUty there is no difference between the 

 two theories for mutations vary in their effects, some gene changes 

 leading to gross modifications of the plant, others leading to infin- 

 itesimal effects, and the two theories become theories regarding 

 the length of time involved in the process of changing teosinte to corn. 



If corn has been developed from teosinte by the selection of hun- 

 dreds of favorable fortuitous gene changes the period of time involved 

 is such that the complete iuterfertihty of the two genera needs 

 explanation, for in such vast periods of time iucompatibilities should 

 have arisen in chromosome configuration such as translocations, 

 inversions, deletions, polysomics, and polyploids that would have 

 limited conjugation and resulted in steriUty. The first explanation 

 of the persisting fertility of these genera when hybridized is that they 

 have remained an interbreediug group from the beginning with 

 man ever preserving corn, nature preserving teosinte, while the 

 LQtermediates fell by the wayside. Aside from the fantastic nature 

 of this assumption, it finds a contradiction in the morphology of the 

 chromosomes of the t'\vo genera. 



When observed at the appropriate stage of cell division the chro- 

 mosomes of both teosinte and of corn are seen to have distinctive 

 enlargements, globose in shape, distributed along their length. 

 These swellings have been termed "knobs" and they are becoming 



