52 



THE AMEBIC AI^^ NATURALIST [Vol.LVI 



characters of hermaplirodites in terms of successive 

 simple mutations in genes. But to interpret male 

 and female forms with observed differences in number or 

 size of chromosomes and with sex-linked inheritailce re- 

 quires comparison with mutations in which the unit of 

 change is a whole chromosome or section of chromosome 

 instead of a single gene. Such mutations can be under- 

 stood in terms of the action of component genes as fol- 

 lows. Linkage experiments show that the various kinds 

 of genes are distributed pretty much at random among 

 the various chromosomes and along each chromosome. 

 But since the number of genes with a given tendency is 

 relatively small, any particular small section of chromo- 

 some might not contain these genes in the same propor- 

 tion as they exist in the entire complement, and still less 

 would the normal proportion of every kind be present. 

 The loss of a section of chromosome (a condition known 

 as deficiency) would ordinarily remove more minus than 

 plus modifiers (or vice-versa), and since in that case more 

 plus than minus modifiers would remain in action, the 

 grade of the corresponding character would be shifted in 

 a minus direction. This is the interpretation of the fact 

 that a deficiency may cause many character changes, the 

 complex of altered characters being inherited as a domi- 

 nant. When a whole chromosome is lost through non- 

 disjunction, the effects are similar to those in deficiency 

 for a section except that they are greater in degree. 



The way in which genes act together in producing 

 a character, and the relation of the balance of plus and 

 minus modifiers to deficiency or to the absence of a chro- 

 mosome may perhaps be made clearer by an analogy. 

 Let us suppose that a man is an ardent stamp collector, 

 and has accumulated a lot of stamps. These stamps are 

 to represent genes, so their number may be put at 5,000 to 

 correspond roughly to the number of genes in DrosopliiJa. 

 Among the Eussian stamps, especially those of ivccnt 

 issue, there is a very large number of reds, Imt niso ;i fair 

 number of pinks, and even a few whites. The-e ditTer- 

 ences in tint correspond to the plus and miims iiidditiers 

 of a certain character, namely, the redness h'u->ian 

 stamps. Now the stamps of different tints are in some 

 definite ratio, whatever that ratio is, and we will call it the 



