VARIATION AND HEREDITY 135 



Mendel's simple theory explains the defi- 

 nite proportions ID -f 2D(R) -f 1R, ob- 

 served when D and R are crossed. It has 

 been tested in various ways, for instance, by 

 crossing D(R) with D or with R, when, as 

 the hypothesis demands, equal numbers of 

 D(R) and D, or of (DR) and R, are obtained. 



In his exceedingly clear exposition of 

 Mendelism (1905), Professor R. C. Punnett, 

 himself a productive investigator, states the 

 characteristic Mendelian result thus : "Wher- 

 ever there occurs a pair of differentiating char- 

 acters, of which one is dominant to the other, 

 three possibilities exist: there are recessives 

 which always breed true to the recessive 

 character; there are dominants which breed 

 true to the dominant character and are there- 

 fore pure; and thirdly, there are dominants 

 which may be called impure, and which on 

 self-fertilization (or inbreeding, where the 

 sexes are separate) give both dominant and 

 recessive forms in the fixed proportion of 

 three of the former to one of the latter." 



Bringing the theoretical interpretation into 

 prominence that is, the theory of gametic 

 segregation, Professor Bateson, the leader of 

 the Mendelian school in Britain, says: 'The 

 essential part of the discovery is the evidence 

 that the germ-cells or gametes produced by 

 cross-bred organisms may in respect of given 



