TYPES OF MENDELIAN HEREDITY 35 



last pair of legs are often thicker and shorter. If 

 many larvae are present, or the food conditions poor, 

 the larvseof rudimentary flies can not stand thecompe- 

 tition and die off, and in consequence the rudimentary 

 class is smaller than expected. The males are fertile, 

 but the females are almost entirely sterile, although 

 rarely one of them may lay a few eggs and some of 

 these hatch. The infertihty is probably due to ab- 

 sence or rareness of mature eggs in the ovaries. 

 There are also other effects than these four men- 

 tioned, all of which are produced by the same factor, 

 and, no doubt, were our knowledge complete, we 

 should find in all mutants many differences in addi- 

 tion to the ones picked out for study and called ''unit 

 characters." DeVries' definition of mutation en- 

 tirely covers this relation; in fact, it even goes 

 further and implies that a single difference may 

 affect the entire organization. Perhaps this does 

 occur, but practically the number of differences that 

 can be observed between a wild and a mutant stock 

 derived from it, is limited. The attack that is some- 

 times made on the unit character hypothesis fails in 

 its intention the moment it is understood that a 

 single factor (difference) has generally not one but 

 many effects. Most workers in Mendelian heredity 

 are fully conversant with these facts. This attack 

 on the unit character conception is usually made 

 by those not familiar with the actual situation and 

 who take the exi)ression unit character too literalh^ 

 It may be conceded that the exi)ression has at times 

 been abused even by some of Mendel's followers. 



