THE CELL-DOCTRINE 139 



recessive are hardly applicable, since the hybrid differs from 

 each of the parents. Segregation not dominance is the essen- 

 tial phenomenon in Mendelian heredity. 



In the foregoing account the results visible in the adult 

 animals have been described without explanation of the 

 causes for this peculiar type of inheritance. Mendel's claim 

 to distinction lies in the fact that he explained the ratios 

 observed in segregation in a manner which has stood the 

 test of later investigation. He knew little of germ-cells. In 

 his day the finer details of fertilization were still undis- 

 covered. He knew merely that there were male germs or 

 pollen grains and female germs or ovules. His explanation 

 of segregation as he would have applied it to the Blue An- 

 dalusian, had he worked with that form, may be stated as 

 follows: The original black individuals, PI in Fig. 19, are 

 pure blacks, since they arise from a black ancestry and give 

 only black descendants when bred among themselves. The 

 same holds for the Pi whites. The symbol B B may be used 

 to designate the black adults and W W the whites. The 

 doubling of the letters indicates that the individuals arise 

 from a double parentage. The blue or hybrid would then 

 be designated as B W. In the figure referred to, the squares 

 stand for adult individuals, the circles for germ-cells. The 

 latter are represented with the character B or W taken 

 once, so that the union of two germ-cells gives the B B, 

 B W, or W W of the adult formula. We are now in 

 a position to understand Mendel's explanation of segre- 

 gation. 



The PI Blacks (B B) arise from germ-cells carrying black; 

 and can produce only germ-cells carrying black; the PI 

 Whites (W W) only germ-cells carrying white. The hybrid, 

 being a union of white-carrying and black-carrying germs, 

 is B W. The problem now arising is what kind of germ- 

 cells will the hybrid (Fi) produce. Will they carry both 

 B and W or will they carry B or W, the one to the exclusion 

 of the other? The first hypothesis does not aid us in explain- 



