264 HEREDITY AND ENVIRONMENT 



and chromosomes come in equal numbers 

 from both parents ; both maternal and paternal 

 factors and chromosomes pair in the zygote 

 and separate in the gamete as shown in Fig. 

 56 and Fig. 57; and so far as is known the 

 chromosomes are the only portions of the germ 

 cells which fulfil these conditions. Further- 

 more there is much additional evidence that 

 the chromosomes are especially concerned in 

 heredity, as was pointed out in the last lecture, 

 and it is not reasonable to suppose that this 

 remarkable coincidence between the distribu- 

 tion of Mendelian factors and of chromosomes 

 is without significance. 



Of course Mendelian factors are not the only 

 factors of development but merely the differ- 

 ential factors which cause, for example, one 

 guinea-pig to be white and its brother to be 

 black. Very many factors are involved in the 

 production of white or black color but there is 

 at least one differential factor for every unit 

 character and this alone is the Mendelian fac- 

 tor. Of course there is no such thing as a 

 "sex-producing chromosome," sex being the 

 result of the interaction of many intrinsic and 



