i] and its Consequences 13 



draught and a white one (Fig. 2. I). Supposing the black 

 to be a dominant the fact may be represented by putting it 

 on the top. When segregation of the allelomorphs, black- 

 ness and whiteness, takes place in gameto-genesis, the 

 germ-cells of the cross-bred are again bearers of blackness 

 or of whiteness, and it may readily be shown experimentally 

 that the results of their various random combinations give 

 rise to the ratios stated above. 



The fact of segregation was the essential discovery 

 which Mendel made. As we now know, such segregation is 

 one of the normal phenomena of nature. It is segregation 

 which determines the regularity perceptible in the here- 

 ditary transmission of differences, and the definiteness or 

 discontinuity so often conspicuous in the variation of animals 

 and plants is a consequence of the same phenomenon. 

 Segregation thus defines the units concerned in the consti- 

 tution of organisms and provides the clue by which an 

 analysis of the complex heterogeneity of living forms may 

 be begun. 



There are doubtless limits beyond which such analysis 

 cannot be pursued, but a vast field of research must be 

 explored before they are reached or determined. It is likely 

 also that in certain cases the units are so small that no 

 sensible segregation can be proved to exist. As yet, how- 

 ever, no such example has been adequately investigated ; 

 nor, until the properties and laws of interaction of the 

 segregable units have been much more thoroughly examined, 

 can this class of negative observations be considered with 

 great prospect of success. 



The dominance of certain characters is often an impor- 

 tant but never an essential feature of Mendelian heredity. 

 Those who first treated of Mendel's work most unfortu- 

 nately fell into the error of enunciating a " Law of Domi- 

 nance" as a proposition comparable with the discovery of 

 segregation. Mendel himself enunciates no such law. 

 Dominance of course frequently exists. The consequences 

 of its occurrence and the complications it introduces must 

 be understood as a preliminary to the practical investigation 

 of the phenomena of heredity, but it is only a subordinate 

 incident of special cases, and Mendel's principles of inherit- 

 ance apply equally to cases where there is no dominance 



