322 



The Journal of Heredity 



Linkage a Limiting Factor 



These assumptions are based on in- 

 dependent inheritance. The assump- 

 tion of independent inheritance is no 

 more justified than the assumption of 

 as few as 20 characters. It is probable 

 that these factors are hnked in groups 

 and that the groups are fewer than 20 

 regardless of the total number of fac- 

 tors. This assumption, while decidedly 

 influencing the results obtainable, does 

 not change the method of procedure. 

 If linkage is presumed, it is logical to 

 presume that some of the objectionable 

 characters of Lion are linked with the 

 smooth-awned factor. If this is the 

 case it limits the achievement of all 

 methods of breeding and if the linkage 

 cannot be broken down the best that 

 can be achieved will be smooth-awned 

 barleys homozygous for the factors of 

 Manchuria except those with which the 

 smooth awn is linked in Lion. This 

 point, if it is the limit, should be more 

 easily reached by back-crossing than by 

 selection from an extensive F= genera- 

 tion. 



There is no reason to believe that 

 linkage is at all an insurmountable 

 barrier. Other smooth-awned parents 

 could be used in which more desirable 

 characters might lie linked with smooth 



awns than in Lion. Also linkage is 

 far from absolute. In the continued 

 back-crossing there is opportunity for 

 cross-overs to break up the undesirable 

 group. As the Manchuria parent is 

 repeatedly used, cross-overs must in- 

 clude desirable characters, which is not 

 necessarily the case in the later genera- 

 tions of the heterozygous elements of 

 the cross when not back-crossed. In 

 many plants the evidence of linkage is 

 so slight as to indicate a very great 

 number of cross-overs. 



Whatever the explanation, it would 

 seem that on this project the easiest 

 approach to the point limited by link- 

 age is by back-crosses and that the 

 greatest opportunity for profitable 

 cross-overs is afforded by the same 

 method. In the smooth-awned project 

 the smoothness of the awns is not being 

 modified by the repeated back-crossing. 

 The F^ plant segregated out in 1922 

 had awns as smooth as those of the 

 parent used in 1920. 



The writers have one or two other 

 breeding projects where the same 

 method is being used and it would 

 seem especially serviceable in such 

 problems as changing the color of 

 otherwise desirable varieties and in 

 securing earliness while retaining the 

 varietal characters. 



Group, Race, and Nation 



The Group Mind, a sketch of the 

 principles of collective psychology 

 with some attempt to apply them to 

 the interpretation of national life 

 and character, by William Mc- 

 DouGALL, F. R. S., Professor of 

 Psychology at Harvard University. 

 Pp. 418. New York, G. P. Put- 

 nam's Sons, 1920. 



One of the distinguishing features of 

 Dr. McDougall's extremely interesting 

 book is the sound eugenic theory which 



runs through it. The eugenist must be 

 profoundly concerned with many of 

 the facts of collective psychology, since 

 he is dependent on them to get carried 

 into effect the principles which he 

 realizes as essential in any permanent 

 and good government. Dr. IMcDougall 

 has, from the psychological side, made 

 many helpful contributions to eugenics, 

 and the present volume, which is a sort 

 of sequel to his widely-known Intro- 

 duction to Social PsycJiology, deserves 

 wide reading. — P. P. 



