360 THE AMERICAN NATURALIST [Vol. XL VI 



acters. They are also from the economic standpoint 

 more important. Their careful study is therefore desir- 

 able. Several years ago I undertook the study of size 

 inheritance in rabbits. I found that when rabbits of un- 

 equal size are mated, the young are of intermediate size, 

 i. e., neither large nor small size dominates in the cross. 

 Further, segregation does not apparently occur among 

 the grandchildren, for these vary about the same inter- 

 mediate mode as the children, though somewhat more ex- 

 tensively. My conclusion was that the inheritance in 

 such cases is non-Mendelian, since neither dominance nor 

 segregation occurs. I called it blending. The experi- 

 ment with rabbits has been repeated on a much larger 

 scale by my pupil, Mr. E. C. MacDowell. He finds, how- 

 ever, that the variability of the grandchildren is consid- 

 erably greater than that of the children, though it 

 seldom extends far enough to include the extreme condi- 

 tions found in the grandparents. This result is con- 

 firmed by observations upon ducks made by Dr. Phillips. 

 It is evident therefore that size is not a simple unit-char- 

 acter, for there is no dominance and no evidence of seg- 

 regation other than the increased variability of the sec- 

 ond hybrid generation. But cases of this kind have 

 recently been interpreted as involving multiple unit char- 

 acters and so as possible Mendelian. This interpretation 

 has been suggested by interesting observations made by 

 Nilsson-Ehle on color-inheritance in oats and wheat. 



In crossing colored with uncolored varieties he ob- 

 tained inheritance ratios of 15 : 1 or 63 : 1, instead of the 

 usual 3 : 1 of colored to uncolored progeny in the second 

 generation from the cross. The ratios obtained in these 

 exceptional cases were such as should occur when two or 

 three independent unit-characters are involved in a cross. 

 But Nilsson-Ehle could discover only a single kind of color- 

 production. The conclusion which one naturally draws 

 from these facts is that the color factor in these cases 

 was localized in two or three distinct bodies independent 

 of each other in their splittings and migrations during 

 cell division. Now Nilsson-Ehle argues with much plaus- 



