430 Stout: The Origin of Dwarf Plants 



phasizes this view in the preface of the English translation of 

 Die Mutationstheorie (1909). 



It has also been shown that numerous cases of hybridization 

 result in an increased variability in the F2 generation over that of 

 the parents, in the production of intermediate forms, and in unex- 

 pected ratios. The Mendelian factorial hypothesis, which at- 

 tempts to explain such phenomena, assumes that the difficulties 

 of analysis in terms of unit characters indicate that characters 

 are themselves the results of the combined action or interaction 

 of hereditary units called factors to which various values can be 

 attributed as seems necessary. There are some striking cases 

 of reversion resulting from crosses that seem to support this view; 

 as, for example, the production in sweet peas of the reversionary 

 purple bicolor known as "Purple Invincible" from crosses be- 

 tween certain white flow^ering strains of "Emily Henderson" 

 (Bateson, 1913, Chap. V)- The developments of the Mendelian 

 theory have led, however, to various and conflicting conceptions 

 regarding the nature of the hereditary units and of the funda- 

 mental processes of mutation or discontinuous variation. 



De Vries on the one hand considers that mutability is due to 

 lability of hereditary physiological units; these change from 

 stable to labile, from active to inactive (or latent) or even to a 

 semilatent condition, giving irregularities of expression both in 

 inbred and crossbred lines of progeny. Only in the rare cases of 

 progressive mutation are new characters added and in none of the 

 mutations is complete loss of hereditary qualities essential. The 

 units of De Vries assume different degrees of activity; they are 

 not uniform and consistent units in their influence. 



In marked contrast to this view is the conception of Bateson 

 that all variation is due to the presence or absence of "unit factors", 

 that all true mutation is due to loss of factors, and that cases of 

 discontinuous variation resulting from crosses are reversions which 

 are due to recombination of factors. This view minimizes the 

 occurrence of latency, ignores lability and insists on a rather 

 rigid unity of assumed hereditary units, not in any sense compar- 

 parable with the appearance of the visible characters of plants. 



These two very different interpretations indicate clearly the 



