180 INBREEDING AND OUTBREEDING 



favorable to the best development of the organism. We 

 do not know, but we may hazard a guess. The repeated 

 appearance and disappearance of certain mutations is 

 merely a type of variability which has probably been a 

 constant feature of the organism for a long period and 

 has been subjected to natural selection in the same way as 

 any other character. In other words, may not the ten- 

 dency to produce dominant unfavorable variations have 

 been reduced to the minimum by natural selection 1 ? Con- 

 versely, a tendency to produce unfavorable recessive mu- 

 tations has been tolerated because the latter are pro- 

 tected in hybrid combinations by their dominant favor- 

 able allelomorphs. Whether this be true or not, there cer- 

 tainly is a strong tendency for dominant unfavorable 

 variations to be eliminated, because they are constantly 

 subjected to natural selection ; while dominant favorable 

 variations, whenever they occur, replace former charac- 

 ters, and become part of the stock in trade of the organ- 

 ism. Recessive mutations, on the other hand, whether 

 favorable or unfavorable, cannot compete for place with 

 natural selection as the judge unless the proper mating 

 brings them into the homozygous condition. If through 

 continuous cross breeding this does not occur, they may 

 be carried on for countless generations family skeletons 

 hidden by the phenomenon of dominance. 



The relation of these reflections to heterosis is just 

 this : If any individual is deficient and handicapped in its 

 hereditary make-up, there is a good chance that this de- 

 ficiency will be supplied when it is crossed with other 

 individuals, because all are not apt to be wanting in the 

 same things. What one lacks is supplied by the other 

 and conversely. In other words, there is a pooling of 



