188 Life and Death, Heredity and Evolution 



race, white in the other (see Figure 51, P). Now we know 

 that in each individual before mating, one chromosome of 

 each pair is gotten rid of, the half nucleus that remains con- 

 taining but a single chromosome of this pair; and that this 

 half nucleus divides to form the migratory and stationary 

 half nuclei of that individual; so that migratory and sta- 

 tionary half nuclei contain the same set of chromosomes. In 

 the one race they will, of course, therefore both contain a 

 black chromosome, in the other a white one. When the ex- 

 change and union of half nuclei occur, the resulting nucleus 

 contains one white chromosome, one black (Figure 51, F 1). 

 This will happen in every pair of the two races that mate 

 together; every family will have a white-black pair of 

 chromosomes. 



In ordinary fission, however, the black and white do not 

 separate, but each merely splits, so that all the individuals 

 produced have this white-black chromosome pair (F 1). 

 This is also just the situation of affairs in the first genera- 

 tion of offspring (the "F 1 generation") from a cross in 

 higher organisms; this F 1 generation is composed of indi- 

 viduals that are alike with respect to their characters. 



But suppose that after a long series of generations, the 

 white-black individuals mate among themselves (as at P 2, 

 Figure 51). What will happen? 



In all the individuals, one of the two chromosomes of this 

 pair will be gotten rid of at the second maturation division, 

 leaving the other. In half the cases it will be the black 

 chromosome that is left ; in half the white one. That is, half 

 the individuals will have black chromosomes in their migra- 

 tory and stationary nuclei, while half will have white ones. 



Of those that contain black chromosomes, half will mate 

 with other individuals that contain black ones, half with 

 those that contain white ones. Similarly, of course, of those 



