REPRODUCTION AND HEREDITY 123 



zygote that the hybrids produced in the cross-breeding 

 just described are developed. Being hybrids they are hetero- 

 zygotes, their composition as regards the characters under 

 discussion being represented by Ss, pure prunaria being 

 ss, and pure sordiata SS. In these pure strains all the 

 mature germ-cells produced in any individual carry the 

 factor for one character or the other. But in the reducing 

 divisions which result in the maturation of the hybrid's 

 germ- cells, the pairing of the chromosomes brings S always 

 alongside s ; then they part and go into different gametic 

 nuclei (see Fig. 32, b, c). So the hybrid produces two 

 kinds of germ- cells each of which must carry either S or 

 s but cannot carr}* both. Therefore the pairing of such 

 hybrids may be represented graphically by 



S 5 



and if each individual produces the two kinds of gametes 

 in equal numbers any gamete will have two chances of 

 meeting in fertilisation a gamete of the opposite kind to a 

 single chance of meeting one of its own. Hence, in families 

 of the second generation resulting from the pairings of 

 hybrids among themselves, half the population may be 

 expected to be hybrid, a quarter to be of one pure strain, 

 and a quarter of the other. And this is as a rule closely 

 approximate to the actual result of breeding experiments. 



The same principles of inheritance are illustrated, though 

 with an interesting difference in detail, in another common 

 British geometrid, the " Pepper and Salt " Moth {Amphi- 

 dasys betularia). This insect (Plate IV, B) derives its common 

 name from the pale grey colour of its wings, traversed by 

 interrupted black bands which vary considerably in extent 

 and degree in different individuals. The species has, how- 

 ever, a well-marked variety (doubledayarid) in which the 

 black scaling is so heavy that the wings present a con- 

 tinuously sooty aspect. It is found that w^hen typical 

 betularia is mated with doubledayaria all the hybrid 



