22O T. II. MORGAN AND SABRA COLBY TICK. 



linked to other characters these deficient ratios, while unfor- 

 tunate, present no serious difficulties, but in experiments in which 

 it is necessary to determine accurately the linkage ratios the 

 difference becomes serious. 



The rudimentary and strap-winged mutants have been used 

 to test the effects of crowding due to mass breeding, because they 

 have been found to show such effects more than any other stocks. 

 In the case of other mutants that give, even in mass cultures, 

 nearly the theoretical values, the method which we now generally 

 employ of breeding by pairs in large culture bottles with plenty 

 of food eliminates almost entirely disturbances due to viability. 



