No. 591] SHORTER ARTICLES AND DISCUSSION 



183 



through crossing, others as due to the introduction of modify- 

 ing factors by means of the cross. "Whichever view is adopted, 

 the fact is perfectly clear that modification of single Mendeliz- 

 ing characters occurs in cross-breeding. Under continuous in- 

 breeding we should expect that single Mendelizing characters 

 (within single lines but not within an entire inbred population) 

 would attain a condition as devoid of variability as it is possible 

 for them to attain and observation confirms this expectation. 



As regards characters which "blend" in heredity, these are 

 not inherited as single characters ; they do not Mendelize in the 

 ordinary acceptation of the term. The characters of the respec- 

 tive parent races disappear in the cross, being replaced by a 

 common intermediate condition or blend. This blend persists 

 into the F 2 and later generations but with a certain amount of 

 variability which is at a maximum in F, and beyond that point 

 tends to disappear in the absence of any special selection. It 

 points to imperfection of the blending process or, in the view 

 of those who prefer a Mendelian terminology for such cases, it 

 points to plurality of factors determining the character. All 

 the cases with which Walton has dealt in the paper under re- 

 view are cases of blending inheritance and as regards them it is 

 true, as already indicated, that continuous inbreeding tends to 

 the production of a more varied population (but not of more 

 variable separate lines) whereas cross-breeding tends to produce 

 a less variable population (devoid of differences between fami- 

 lies) but nevertheless a population more variable than the single 

 lines of a self fertilizing or constantly inbred population. 



W. E. Castle 



Busset Institution 



