TYPES OF MENDELIAN HEREDITY 35 
last pair of legs are often thicker and shorter. If 
many larve are present, or the food conditions poor, 
the larveeof 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 infertility 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 expression unit character too literally. 
It may be conceded that the expression has at times 
been abused even by some of Mendel’s followers. 
