GENETICS: H. /. MULLER 
625 
than beaded have been found to intensif)^ or inhibit the development 
of the latter, and there is besides, as Dexter has shown, a factor in 
the second chromosome of the selected beaded stock itself which has no 
visible effect other than to increase the degree of beading. This must 
have arisen by mutation and have been perpetuated in the process of 
selection. Crosses made in the course of the present work have shown 
that this intensifier, l-Qdj is partially dominant, but is not a lethal, and, 
in contrast to the other factors involved in beaded stock, that it exists 
here in homozygous condition. At least one mutant factor has also been 
found, by the writer, which can produce a character similar to beaded 
even when the factor Bd' itself is not present; this mutant is not or- 
dinarily present in beaded stock, however. All the facts of the present 
section may be summed up in the single generalization that beaded is a 
character depending upon a developmental reaction that is readily 
modifiable. 
The complete formula of the selected beaded race, representing ail the 
pairs of factors wherein it differs from the wild type, may now be given: 
Ib^ Chii 1 
10. The beaded case illustrates to great advantage the danger of 
confusing characters with gens and of drawing radical conclusions 
concerning the behavior of gens on the basis of uncritical experiments. 
The work of the first four years upon the inheritance of beaded wings 
gave evidence which would to many have appeared most elaborate and 
convincing, that the hereditary material in this case was fluctuating 
and miscibie, consisting of vague and plastic ''tendencies," rather than 
definite physical particles. Precise analysis, of a sort comparable to 
that of chemistry, has, however, been possible here, and it has demon- 
strated that a very different set of processes from those that might 
have been imagined is responsible for the peculiar results — ^processes 
which in their essence conform strictly to the genotype conception. 
It will accordingly be necessary in other cases also not to accept 
evidence apparently in favor of factor incons::a,ncy until factorial analy- 
ses of a similarly rigorous character have proved such an interpretation 
to be correct. A similar criticism applies to the acceptance of results 
that seem to be non-Mendelian ; and also to the incautious estimation 
of apparent mutations at their face value. 'Non-Mendelian' results 
of all kinds and also 'mutations' may be prearranged and brought 
about at will with the beaded flies, but here analysis has made the 
