86 PHYSICAL BASIS OF HEEEDITY 



in D. husckii, Sturtevant in D. repleta). Nabours has 

 found a case in one of the grouse locusts, and Castle and 

 Wright in rats. In the silk-worm moth, Tanaka has found 

 one group of linked characters. In poultry Goodale has 

 found one case. In the moths and poultry it appears 

 that linkage is complete in the female, incomplete in the 

 male. In this respect the situation is the reverse of 

 that in Drosophila, There are some other cases where 

 linkage is suspected but uncertain. 



The fact that relatively so few cases of linkage have 

 been as yet reported is due in part to the fact that in 

 most species the heredity of only a very few charac- 

 ters is generally known. Where more are known each 

 has as a rule not been examined in relation to all the 

 others, so that even if some of the factors were linked it 

 would not have been found out. Furthermore, in Mendel- 

 ian crosses, the practice of mating i^i'5 instead of back- 

 crossing, tends to conceal the linkage phenomena if pres- 

 ent. The fact of greatest significance, however, is that 

 the number of cases of linkage is steadily increasing as 

 the inheritance of more characters in each species is 

 becominof known. 



