Ch. VI, 8] HEREDITY AND VARIATION 309 



mosome mechanism earlier described, heredity should oc- 

 cur. Indeed, on this basis, offspring should repeat their 

 ancestors exactly, and the scheme leaves no room for vari- 

 ation at all. 



The student will note the phrase "like its ancestors," 

 not "like its parents." It is a matter of popular knowledge 

 that family characteristics often skip a generation, or several 

 for that matter; and children thus show features of their 

 grandparents intermingled with those of their parents. 

 Our knowledge of this subject is now firmly grounded, thanks 

 to the labors of Mendel and his many modern successors in 

 experimental biology. As a result it seems clear that the 

 characters or features which make up an individual, and 

 which are built by its cytoplasm under control of its chromo- 

 somes, are not indefinite in number and kind, as popularly 

 imagined, but are definite in both respects. In other words, 

 an individual consists of a definite, though great, number 

 of ultimate irresolvable unit characters, of which it forms 

 a kind of mosaic. Furthermore, each such unit character 

 is apparently represented in the chromosomes of all of 

 the cells by some kind of determiner which controls the 

 construction of that character by the cytoplasm, though 

 whether this determiner be some material carrier, some kind 

 of register, some form of model, some type of enzyme, or 

 some other entity, is not known. Accordingly, the ferti- 

 lized egg cell, and every body cell formed therefrom, having 

 its two sets of chromosomes, must contain two sets of all 

 the determiners necessary to construct that kind of organ- 

 ism ; or in other words every kind of character of an organism 

 is represented in duplicate in every one of its body cells, one 

 determiner being contributed by each parent (see the dia- 

 grammatic Fig. 219). Now arises the question : How do these 

 duplicates behave with respect to one another during the 

 development of the cell, and what determines which one is 

 to direct the cytoplasmic construction, and thus determine 

 the character, in any particular case ? On this matter Men- 



