378 THE STUDY OF ANIMAL LIFE CHAP, xix 



be fortuitous, the proportions 1DD + 2D(R) -f 1RR 

 must result. 



As Professor Punnet t puts it : 



" A blue hen is producing equal numbers of ' black ' and ' white ' 

 eggs let us say 2n of each. To fertilise these eggs are brought large 

 numbers of spermatozoa of the two sorts, black and white, in equal 

 numbers. Every black egg, then, has an equal chance of being 

 fertilised by a black or a white spermatozoon. In the former case 

 it will form a black, and in the latter a blue, bird. From our 2n 

 black eggs we shall obtain n black and n blue birds ; that is to 

 say, the mating of blue with blue must, on the assumption of the 

 purity of the gametes, give black, blue, and white in the ratio 

 1:2: 1." 



The theory of gametic segregation is at the centre of 

 Mendelian theory. As Professor Bateson says : 



" The essential part of the discovery is the evidence that the 

 germ-cells or gametes produced by cross-bred organisms may in 

 respect of given characters be of the pure parental types, and con- 

 sequently incapable of transmitting the opposite character ; that 

 when such pure similar gametes are united in fertilisation, the 

 individuals so formed and their posterity are free from all taint of 

 the cross ; that there may be, in short, perfect or almost perfect 

 discontinuity between these germs in respect of one of each pair 

 of opposite characters." 



5. Social and Ethical Aspects. All the important 

 biological conclusions have a human interest. 



The fact of organic continuity between germ and germ 

 helps us to realise that the child is virtually as old as the 

 parent, and that the main line of hereditary connection 

 is not so much that between parent and child as " that 

 between the sets of elements out of which the personal 

 parents had been evolved, and the set out of which the 

 personal child was evolved." The main line," Galton 

 says, " may be rudely likened to the chain of a necklace, 

 and the personalities to pendants attached to the links." 

 To this fact social inertia is largely due, for the organic 



CJ */ O 



stability secured by germinal continuity tends to hinder 

 evolution by leaps and bounds either forwards or back- 

 wards. There is some resemblance between the formula 

 of heredity and the first law of motion. The practical 

 corollary is respect for a good stock. 



