186 EVOLUTION 



Mendel's simple theory explains the definite 

 proportions ID + 2D(R) -f 1R, observed 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 : 

 " Wherever there occurs a pair of differen- 

 tiating characters, 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 therefore 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 



