100 THEORY OF EVOLUTION 



case has a sufficient number of characters been 

 studied to show whether there is any corre- 

 spondence between the number of hereditary 

 groups of characters and the number of chro- 

 mosomes. In the fruit fly, Drosophila ampelo- 

 phila, we have found about 125 characters that 

 are inherited in a perfectly definite way. On 

 the opposite page is a list of some of them. 



It will be observed in this list that the charac- 

 ters are arranged in four groups, Groups I, 

 II, III and IV. Three of these groups are 

 equally large or nearly so ; Group IV contains 

 only two characters. The characters are put into 

 these groups because in heredity the members 

 of each group tend to be inherited together, 

 i.e., if two or more enter the cross together they 

 tend to remain together through subsequent 

 generations. On the other hand, any member 

 of one group is inherited entirely independently 

 of any member of the other groups ; in the same 

 way as Mendel's yellow-green pair of charac- 

 ters is inherited independently of the round- 

 wrinkled pair. 



If the factors for these characters are car- 

 ried by the chromosomes, then we should ex- 



