SUMMARY OF CURRENT RESEARCHES. 51 



of one parent (A x B = A 2 ). (3) The cross-bred offspring, e.g. of 

 spotted, black-and-white mice and albino mice, may have a charactei 

 entirely different from either parent, " a character of its own " 

 (A X B = C). (4) The cross-bred offspring, e.g. of white and bnff 

 pigeons, may resemble an ancestral form, may exhibit " reversion " 

 (A X B = R). (5) But, finally, as with grey and white mice, the off- 

 spring may show patently the character of one parent only (A X B = 

 A (B) or B (A), according to the law of dominance. 



(8) Purity of the Germ-cells. - — The great discovery of Mendel is 

 this : The hybrid, whatever its own character, produces ripe germ-cells 

 which bear only the pure character of one parent or the other. The 

 hybrid A (B) or B (A) will have germ-cells, bearing either the character 

 A or the character B, hut not both ; and A's and B's will he produced 

 in equal numbers. This is the law of " segregation " or the law of the 

 "purity of the germ-cells." " It bids fair," Mr. Castle says, " to prove 

 as fundamental to a right understanding of the facts of heredity as is 

 the law of definite proportions in chemistry. From it follow many 

 important consequences." 



A first consequence is polymorphism of the second and later hybrid 

 generations, as may be represented in the following scheme : — 



A x B = A (B) or B (A). 



A (B) x A (B) = (1) A set of pure A's, which if inbred will 



breed true to that character ever 



afterwards. 



(2) A set of similar pure B's approximately 



equal in number to the pure A's. 



(3) A third set of A B's like those of the 



first hybrid generation. 



If this be expressed in terms of the germ-cells, it seems to mem 

 this : — A (B) produces germ-cells bearing either the character A or the 

 character B. If a male germ-cell A meet a female germ-cell A, the 

 result is an offspring pure A ; if a male germ-cell B meet a female 

 germ-cell B, the result is an offspring pure B. As these pure A's and 

 pure B's occur in approximately equal numbers, the inference is pro- 

 bably correct that the original hybrid A (B) produces two equal sets of 

 gametes, dominantly A's and B's. There is one chance each for the 

 combinations A A and B B, and two chances for the combination A B. 

 And the whole progeny tends to occur in the proportions 1 A 4- 2 A B 

 + 1 B. As a mattter of fact this does occur. 



In his experiments with the Chinese primrose (Primula sinensis) 

 Bateson produced an unfixable hybrid, " Giant Lavender," by crossing 

 a magenta-red with a white flowering variety tinged with pink. This 

 hybrid constantly produces plants bearing magenta-red and white 

 flowers respectively, as well as other plants bearing lavender flowers in 

 the proportion of 1:2:1, the exact numbers being 12 : 23 : 11 and 

 9:20:9. 



In cases of complete dominance, only two categories of offspring 

 will be recognisable, and these will be in the ratio of 3:1, but the 

 larger group on further breeding breaks up into two classes : first, 



E 2 



