THE PURE LINE HYPOTHESIS. 541 



chance. Thus there is separation or segreg"ation in the mature 

 germ-cells of the two slightly different factors, and, provided it 

 is actually a matter of chance which of the two homoloo^ous 

 chromosomes remains in the mature germ-cell, half^df the eggt-cells 

 will contain the factor originally derived from the grandparental 

 seed-plant, and half from the grandparental pollen-plant. In a 

 similar manner, half of the male germ-cells will contain the factors 

 derived from the grandparental pollen-plant and half from the 

 grandparental seed-plant. 



On self-fertilising the plants thus containing two kinds of 

 each of the sexual cells, we should obtain three kinds of fertilised 

 egg-cells, namely: (i) Those containing two homologous chromo- 

 somes derived from the grandparental seed-plant; (2) those 

 containing one chromosome from the grandyjarental seed-plant 

 and one from the grandparental pollen-plant; and (3) those con- 

 taining two Jiomologous chromosomes from the grandparental 

 pollen-plant. The proportional occurrence of these three dif- 

 ferent kinds of fertilised germ-cells would be i : 2 : i respectively, 

 which is, of course, the familiar Mendelian relationship. 



Thus independently of any replacement of the factors for 

 redness by its allelomorph, say the factor for whiteness, we should 

 obtain the JNIendelian relationship with reference to any slight 

 differences in the two factors for redness in the two grandparents, 

 proznded that these factors ivere unable to influence each other, 

 and were incapable of each assuming an intermediate or mean 

 nature. It is realised that the detection of such Afendelian 

 relationship for slightly different factors for redness would not 

 be very easy ; but no clear evidence o;f its occurrence in the breed- 

 ing of numerous families has been seen, and the obvious inference 

 is that the two slightly different factors do mutually influence each 

 other and become alike and intermediate in nature. 



It is still quite debatable whether all hereditary characters 

 are carried by the chromosomes, and it has been plausibly sug- 

 gested by C. E. Walker* and others that " individual " characters 

 are carried by the chromosomes and " racial " characters by the 

 cytoplasm. It would appear that in the latter case the blending 

 type of inheritance is more probable, while in the former case the 

 Mendelian or the non-blending type may occur. 



If it is advisable to retain the conception of factors or discrete 

 potentialities in the germ-cells, we may, in the case of hereditary 

 characters carried by the nucleus, express the relationship between 

 non-blending and blending inheritance in the following way. In 

 non-blending inheritance the mutually replaceable factors are of 

 such a nature that they are unable, when combined in the same 

 nucleus, to influence each other and each to become modified into 

 a factor with a nature intermediate between the natures of the 

 two original factors ; while in the case oif blending inheritance 

 such a modification of the two factors does occur (see plate). 



* Walker. C. E. : " Hereditary Characters and their Modes of Trans- 

 mission." London, I9,IQ. 



