Hybrid Vigor 179 



are widely variable. In this case, however, the average 

 size of the whole population is distinctly different in the 

 different generations. In such a matter as size the 

 hybrid vigor manifestations would he superimposed upon 

 the cumulative factor manifestations. 



It might appear unsatisfactory and arbitrary to assume domi- 

 nance of factors as essential to explaining hybrid vigor, and lack 

 of dominance in the case of cumulative factors. It is quite likely, 

 however, that the fundamental and ''natural" distinction between 

 the two mechanisms lies in this very point. Where a number of 

 factors interact in affecting some quantitative character and those 

 factors show lack of dominance, a cumulative factor mechanism 

 is thereby set up. A similar interaction where the factors are 

 dominant brings into play the hybrid vigor mechanism. There 

 is a difference between the two mechanisms simply because some 

 factors show dominance and others do not. 



This idea may be reinforced by the following theoretical sug- 

 gestion. Where the environment (using the term in its widest 

 sense) imposes no limitation upon the degree to which a character 

 may be expressed, it follows that two doses of a factor must have 

 twice the effect of one; dominance is lacking. Where the environ- 

 ment limits the expression of a character, and one dose of a factor 

 results in a development of the character to this limit, two doses 

 can affect nothing more; dominance is present. Furthermore, 

 these environmental limitations may shift as the environment 

 changes. Such an environmental shift could affect in no way the 

 degree of development of those characters in connection with 

 which there is no dominance, but would be expected to affect the 

 degree of development of those characters where dominance 

 occurred. According to this idea, we should expect non-domi- 

 nance or cumulative factor characters to be of such a sort that the 

 environment never affects the degree of their development; while 

 dominance or hybrid vigor characters would be those which envi- 

 ronmental changes could also modify. lH>r the most part this 

 actually agrees with the facts (see p. 165). Further investigation 

 will doubtless provide a more definite answer on this matter. 



