528 MANUAL OF ELEMENT AR Y ZOOLOGY 



will throw back to the recessive grandparent (in this case, 

 the black rabbit). Thus it comes about that both varieties 

 remain unimpaired within the species, and either will have 

 its effect on the survival of individuals in successive 

 generations. 



These facts are explained as follows. Each germ contains potentially 

 Mend II m a " '^ e characters of an adult organism (p. 191). When 

 germs unite the zygote therefore contains a double set 

 of tendencies. In regard to each character, the germs may have been 

 alike or unlike. If they were alike the zygote is called a homozygote 

 and will show the character they both bore. If they were unlike the 

 zygote is called a heterozygote, and one tendency becomes dominant over 

 the other in its body-cells. At the reduction division in the formation 

 of germs the tendencies again separate. In regard to the character in 

 question, the germs of a homozygote will all be alike, those of a hetero- 

 zygote will be unlike, consisting of equal numbers of both kinds. 

 When heterozygotes breed together, on the average half the offspring 

 will be formed by the union of unlike germs — that is, will be hetero- 

 zygotes like their parents. Half the remainder will be homozygotes 

 formed by the union of germs bearing the dominant character ; half 

 will be homozygotes formed by the union of recessive germs. All 

 the dominant homozygotes and all the heterozygotes will develop the 

 dominant character. All the recessive homozygotes will develop the 

 recessive character. Thus three-quarters of the offspring will have 

 the dominant character, one-quarter the recessive character. The 

 recessives, if bred with recessives, will all breed true. Of the seeming 

 dominants only one-third — those which are pure dominants — will breed 

 true ; the rest — the heterozygotes — will continue to throw one-quarter 

 of pure recessives. In the following genealogical table G represents 

 a homozygote grey rabbit, B a. black, and (G) a heterozygote grey, 

 and, in order to represent the results of various kinds of matings, it is 

 supposed that in the second generation a brother and sister breed 

 together, and in the third there is one union of brother and sister and 

 two with members of other stocks. 



GxB 



I 



I I I I 

 (G) (G)x(G)(G) 



I 



I II I 



GxG (G)x(G) BxB 



I I I 



nil 1 1 1 1 r m 



G G G G G (G) (G) B B B B B 



There are many difficulties in the way of the theory of 



evolution by natural selection : such, for instance, as the 



