MULTIPLE FACTORS IN MENDELIAN INHERITANCE 183 



short stocky vs. long slender legs of Dexter-Kerry cattle, J. 

 Wilson ('09). 



The evidence presented by plant breeders seems to carry con- 

 viction. It has shown that the assumption of multiple factors 

 is the most simple theory to explain certain ratios in crosses in- 

 volving color and its absence. Similar ratios are also found in 

 crosses involving certain structural characters (ligulae and the 

 arrangement of spikelets in wheat). Now if qualitative char- 

 acters may depend upon multiple factors in certain cases, there 

 is just as good evidence for saying that certain morphological 

 characters depend upon multiple factors. Size crosses differ from 

 crosses involving multiple factors for color and structural char- 

 acters in that 'in the latter eases the color or the structure may 

 be absent, and, so definite ratios can be found; whereas in the 

 former size as such cannot be absent, so in most cases no definite 

 ratios can be found. Otherwise, these two types of characters 

 are strikingly similar. Both show wide variations in the second 

 generation not found in the parental or first generations. Both 

 tj^pes show similar wide variations in the second generation in 

 crosses between certain strains that appear to be alike and in 

 these cases the first generation is like the parents. Both types 

 may show mono-hybrid ratios in the same sorts of characteristics. 

 Any theory to account for the wide variations in the second gen- 

 erations of crosses involving characters which cannot afford defi- 

 nite ratios must also account for this similarity with cases where 

 definite ratios may be found, and again, with cases where sim- 

 ple Mendelian ratios and complex ones are found for the same 

 character differences. Size and Mendelian inheritance are not 

 incompatible, as is shown by many mono-hybrid ratios from 

 crosses involving size. In one case (semi-dwarf peas) it has been 

 shown even by ratios that size may depend upon two distinct 

 Mendelian factors. Surely the most simple theory that can be 

 given to explain this phenomenon of increased variability, which 

 in itself cannot be doubted, being reported by many investiga- 

 tors for so many characters, is that there exist units of inheri- 

 tance, introduced by the parents, to the segregation of which units 

 in the germ cells of the hybrids, the new combinations are due. 



