MENDELISM AND BIOMETRY 209 



characters of an organism are not a complete guide to 

 the nature of its gametes. It is only by careful 

 breeding that we can distinguish the heterozygote from 

 the pure dominant foim to take the simplest possible 

 example of this difficulty. For this reason it has now 

 become the chief business of the student of heredity 

 to determine by experiment what combinations of 

 allelomorphs are present in the gametes of the indi- 

 viduals with which he is working. 



The behaviour of these allelomorphs has now been 

 disentangled in many cases of very considerable com- 

 plexity ; and all such cases as have been so far examined 

 in detail have proved explicable in terms of a larger or 

 smaller number of allelomorphic pairs, all of which 

 obey Mendel's law with the single exception of those 

 cases in which coupling between the allelomorphs of 

 different pairs introduces a slight further complication. 

 Although it is perhaps scarcely probable that Mendel's 

 law will ultimately prove universal in its application, 

 nevertheless the few exceptions recorded by competent 

 observers still require further examination before they 

 can be accepted as invalidating the law in any single 

 instance. 



The question naturally arises as to how far the 

 Mendel ian rule of inheritance agrees with or contra- 

 dicts those estimations of hereditary values which 

 have been arrived at by the labours of the biome- 

 tricians. 



So long ago as 1902 Mr. G. Udney Yule endeavoured 

 with some apparent success to reconcile the Mendelian 

 results with those of biometry. Progress has been 



