86 HEREDITY [ch. 



have been influenced in any way by its association 

 with the other. It has been maintained that the 

 Mendelian categories are not sufficiently definite to 

 allow such a statement to be made with certainty. 

 The Mendelian can only reply, that in the great 

 majority of cases the ' extracted ' pure individuals in 

 the F2 generation do not differ recognisably from 

 the original parents in the characters considered, 

 and that no signs of impurity can be found in later 

 generations. 



There are however instances in which it appears 

 that Mendelian segregation may not be perfect. It 

 has been maintained that an instance of this is 

 provided by hair-length in guinea-pigs. When a 

 long-haired ('Angora') guinea-pig is mated with 

 a short-haired, the F^ offspring are short-haired, 

 shortness being dominant, owing perhaps to the 

 presence of a factor which prevents the growth of 

 the hair after reaching a certain length. But when 

 such Fi (heterozygous) short-hairs were mated 

 together, in addition to apparently pure longs and 

 shorts, animals with hair of intermediate length 

 were produced, and these crossed back with pure 

 long-hairs gave no short-haired young. It is suggested 

 that the long and short characters have become fused 

 in some germ-cells, segregation being incomplete or 

 non-existent, so that germ-cells bearing the mixed 

 character are produced. Again, in a cross between 



