HEREDITY 95 



a pair of Andalusians we get, on an average, one black 

 like the original black of the first cross, two grays or 

 Andalusians like the immediate parents, and one white 

 like the original white of the first cross. Breeding from 

 these again we find (bottom line of diagram) that two 

 blacks bred together give nothing but blacks, the \vhites 

 give nothing but whites, while the grays again give all 

 three kinds in the average proportions of one black, two 

 grays, one white. 



Now the interpretation of this result is as follows. Each 

 separately inheritable characteristic such as the colour 

 of the feathers in the present instance is supposed to be 

 represented in the germ-plasm by a definite part or con- 

 stituent of the hereditary substance, which may conveni- 

 ently be called a "factor." Thus the germ-plasm of the 

 white fowl contains the factor for whiteness, and that of 

 the black fowl the factor for blackness or as we may 

 more shortly call them, white and black factors. 



When a black and white fowl are crossed, as in the 

 first line of the diagram, the resulting hybrid has both the 

 black and white factors, which act together to cause 

 the gray colour of its feathers. In the nuclei of the germ- 

 plasm, however, the black and white factors do not mingle, 

 but may be conceived of as lying separately side by side. 

 When the gametes are formed and this is the crux of 

 ' Mendel's law ' -the two factors separate in such a 

 manner that the number of gametes carrying the black 

 factor is equal to the number carrying the white. No 

 gamete, however, can carry both black and white factors. 

 This process of sorting out of factors in the gametes is 

 spoken of as segregation. 



We are now in a position to understand the result of 

 mating two gray birds together. The gray hens are 



