THE PL-KE LINE 11 VruTlIESlS. 537 



characters and unit-factors which refuse to mingle intimately 

 with each other in sexual reproduction, and the whole of the 

 character, or its allelomorph, is handed down by the j^arent. 1 he 

 conception of unit-characters and Mendelian inheritance has not 

 been elaborated on account of any inherent plausibility, but it has 

 been forced into recognition by the striking results that have been 

 obtained in the breeding of animals and plants. 



4 Blending and Non-Blending Inheritance. 



It must be remembered that the Mendelian relationship has 

 been observed most frequently in the case of the relatively less 

 important characters which distinguish domestic breeds; but the 

 more fundamental characters of natural species do not appear 

 to exhibit this type of inheritance. Here the ofTspring usually 

 have the mean of the characters of the two parents, and on 

 subsequently mating the offspring together, there is no clear 

 segregation or separation of the characters of the grandparents. 

 In other words, the characters of the grandparents have become 

 inseparably blended. 



The strict Mendelians, however, tend to deny the existence 

 of real blended inheritance. They hold, seemingly, that the 

 appearance of blending may result from a mosaic or combination 

 of numerous unit-characters which hang together in groups, and 

 the groups are so related to one another that an apparent blending 

 of characters may occur. 



Morgan points out that the inheritance of characters in 

 groups is to be expected if the factors on which they depend are 

 situated in definite places in particular chromosomes ; but the 

 chromosome mechanism would appear to be relatively too coarse, 

 and the number of chromosoraes infinitely too small, for the pur- 

 pose of attempting to explain the appearance of blending by the 

 meeting together in homologous male and female chromosomes 

 of numerous minute, unmodifiable unit-factors. 



It would appear that we are bound to recognise the existence 

 of genuine blended inheritance, and further, that this type mav 

 concern the more important and fundamental characters of 

 organisms. 



The relationship between these two types of inheritance is 

 not entirely clear, and whether future cytological observations 

 will be able to throw light on the subject is uncertain. It is. how- 

 ever, somewhat significant that the nuclear mechanism which 

 has been shown to occur in the case of some germ cells is oif such 

 a nature as to explain Mendelian segregation and other 

 phenomena associated with non-blending inheritance, on the sup- 

 position that the factors in the fertilised t^^, controlling or repre- 

 senting the characters of the future individual, are lodged in the 

 chromosomes. A possi])le. but not necessarv inference which is 

 suggested bv thi^; coincidence of behaviour between chromosome 

 movements and Mendelinn phenomena is that the factors repre- 

 senting, or controlling', the characters which do blend are not 

 lodged in the chromo<:ome'^, but in the nuclear sap. o ' more likely 

 in the general cytoplasm. 



