122 THE AMEBICAN NATURALIST [Vol. LV 



There seems to be no sure way of distinguisliing be- 

 tween the behavior of characters that appear as the re- 

 sult of recombination and those that result directly from 

 a germinal change. Changes that occur in homozygous 

 strains must be mutations. It is, however, theoretically 

 impossible to be certain that a strain is homozygous re- 

 gardless of the number of generations that it has been 

 selfed and, practically, the criterion of homozygosity is 

 fixed by the accuracy with which comparisons can be 

 made. With quantitative characters in maize it is diffi- 

 cult to detect with certainty differences of less than 10 

 per cent., yet sister progenies of strains that have been 

 selfed for as many as 8 or 9 generations usually show 

 differences too large to be ascribed to chance. This diffi- 

 culty of obtaining uniformity may be due 'to the large 

 number of factors involved, but also may be due to the 

 frequency of minor mutations. If a new character ap- 

 pears in a relatively uniform strain that has been selfed 

 for a number of generations and the character behaves 

 as a simple Mendelian unit, it usually is ascribed to a 

 nmtation. Even in such cases, however, the character 

 may be due to recombination. If the two factors of a 

 dihybrid recessive character arose independently in 

 nearly the same position on homologous chromosomes, 

 the close linkage of the dominant allelomorph of one fac- 

 tor with the recessive allelomorph of the other would 

 long postpone the appearance of an individual with both 

 recessive factors and. when it did appear the departure 

 of its behavior from that of a simple character would be 

 difficult to detect. 



Although the nature of variations does not affect the 

 bearing which the preponderance of recessive characters 

 has on the explanation of heterosis, there are practical 

 as well as theoretical reasons for wishing to know 

 whether new characters that appear in breeding stocks 

 aie mutations or result from the combination of factors 

 already present in the germ plasm. If the undesirable 

 characters that appear from time to time, oven in well- 



