;><) Oil I nheritaiice of Weiijht in PouUnj 



weights by ^, a proportion which experience has shown us to express 

 very nearly the relation between average weights of the sexes in the 

 material we have worked with. 



In Table III we have interpreted the experimental results in terms 

 of the scheme just outlined. It is clear from this table that the scheme 

 covers the facts fairly closely. The chief (liscre[)atice is in I ho figures 

 for the F.. generation where the j)roportion of heavy birds among the 

 pullets is somewhat below expectation. Apart from this the actual 

 results from the various matings, both with regard to mean weight and 

 range of variation, tally closely with the figures deduced theoretically on 

 our hypothesis. It is not impossible that an even closer agreement 

 might be arrived at if the values given to the various factors were 

 altered'. At this stage however we are more concerned in demonstrating 

 that an explanation of these phenomena in terms of definite genetic 

 factors is possible. To determine the action of I'ach factor with pre- 

 cision would be a long and laborious undertaking, nor do we propose to 

 pursue the matter so far. But if our hypothesis is a(le(juate we ought 

 to find birds <if different intermediate sizes breeding true. We should 

 for example be able to establish a breed of the constitution AAbbccDD 

 and of weight grade IflK (Table II). Such a grade is exactly inter- 

 mediate between AABBCCdd and aabbccDD, i.e. between the Cold 

 Hamburgh and the Silver Sebright. We ought also to be able to 

 establish another intermediate race of the constitution aabbCCDD 

 which would breed true to a grade rather above that of the Sebright. 

 Experiments on these lines are already in progi'ess. Moreover, until we 

 have succeeded in establishing such sti-ains we cannot know for certain 

 what amount of variation is due to non-inheritable fluctuations apart 

 from the operation of definite genetic factors which affect weight. In 

 the present paper we have not taken such possible fluctuations into 

 account as there seems no reason why they should not affect all of the 

 birds similarly. 



We alluded above to soiue experiments in which t\ an<l F., birds 

 from the Sebright-Hamburgh cross weie mated with pure Brown 

 Leghorns (p. 31), and we pointed out that in the case of the F^ and 



1 For the sake of simplicity we have conceived of our factors as acting in each case 

 directly on a minimum and constant basal weight. But it is of course possible to regard 

 them as acting also upon the weight resulting from the other factors present. For 

 example we have supposed that a single dose of D gives an increase of 25 % on the 

 minimum weight of 100, whether A, B, or C are present or not. But it may be that D 

 produces a 2.5% increase on whatever is present. And of course it is possible that weight 

 factors ma}' act upon one another. 



