Bearers of Hereditary Characters 201 



(3) If chromosomes bear hereditary characters, and 

 there are only two pairs of chromosomes in a plant, the 

 result would obviously be nothing above the dihybrid 

 ratio. Although the plant might show many more 

 than two Mendelian characters these characters should 

 be linked into two groups and dihybrid ratios result. 

 If trihybrid ratios were obtained, one should have to 

 conclude that something else than two pairs of chromo- 

 somes was involved. It is a mechanical necessity that 

 the ratios should not exceed the number of chromosomes. 

 For example, do we ever have pentahybrid ratios in a 

 plant with four pairs of chromosomes? This question 

 cannot be answered fully for plants, because all the plants 

 used in experiments have an ample number of chromo- 

 somes to account for the highest ratios discovered. In 

 MORGAN'S fruit fly, however, there are actually only 

 four pairs of chromosomes, and in all the great mass of 

 data collected on inheritance in the fruit fly tetra- 

 hybrid ratios occur regularly, but pentahybrid ratios 

 never. 



(4) Finally, the fact that the chromosome hypothesis 

 is coming into more and more use in plant-breeding is a 

 general evidence in its favor. The plant breeder does 

 not usually question whether the theory of inheritance 

 he uses is true or false, but merely whether it is useful 

 or barren in leading to new results or connecting up 

 known facts. On this basis the chromosome hypothesis 

 is proving very useful. 



ARGUMENTS AGAINST THE CHROMOSOME HYPOTHESIS 



(i) Without going into details the general fact may 

 be stated that the chromosome hypothesis does not 



