3 



to need discussion, since, in several recent 

 conversations, I have found the notion to pre- 

 vail that recessiveness is a handicap, and allu- 

 sions based upon the same idea have found 

 their way into print. This view is quite 

 erroneous; not only has the dominant form 

 no advantage in the competition which the 

 newly arisen elementary species must en- 

 counter, but it can be shown that under cer 

 tain conditions the reverse is true. 



If the dominant and recessive forms are 

 equally adapted to the particular environment 

 in which they live, there is absolutely no ad- 

 vantage in favor of either. The second gen- 

 eration of a Mendelian monohybrid contains 

 the same number of pure recessives as of pure 

 dominants, and the heterozygotes continue to 

 produce in each succeeding generation just as 

 many recessives as extracted dominants. The 

 chances that extracted dominants will self- 

 fertilize or that they will cross with other 

 extracted dominants are exactly the same as 

 the chances that recessives will self-fertilize 

 or cross with other recessives. In like manner 

 extracted dominants and recessives will cross 

 with heterozygotes with equal frequency, and 

 the quantitative results in these two cases will 

 be exactly parallel, in one instance giving 

 fifty per cent, of pure dominants, in the other 

 case fifty per cent, of pure recessives. In this 

 equal fashion the struggle will continue in- 

 definitely so long as the premise holds, that 

 the two forms are equally well suited to the 

 conditions under which they must grow. 



The situation is different when natural 

 selection favors one or the other of the com- 

 peting forms. A single extreme case will 

 suffice to demonstrate : Let us suppose that the 

 new form is dominant over its parent, but 

 so poorly adapted to the particular habitat in 

 which it originated that it can not successfully 

 compete with the parent form. All the 



