380 CALVIN B. BRIDGES 
No one can deny that progress under selection is theoretically 
possible by repeated mutation in a single locus. But to accept 
that as the actual mode in any particular case demands specific 
proof. Such a hypothesis should not be considered until it has 
first been demonstrated that the initial constitution of the stock 
was such as to require the assumption of further acts of mutation. 
If a fresh act of mutation is required, then adequate proof must 
be submitted before it can be accepted that this mutation is in 
the particular locus favored rather than in one or another of the 
numerous other possible loci. Our experience with many cases 
of successful selection in Drosophila has been that even in this 
form, where the work is aided by such special features as a very 
small number of chromosomes, by absence of crossing over in 
the male, and by a knowledge of the initial constitution of the 
stocks that is not paralleled in other forms, it is often a matter 
of some difficulty to prove that a particular modification arose 
during the course of an experiment rather than that it was present 
in and introduced through one of the parents. How much more 
difficult, then, would it be to prove in a form where the tests are 
far less precise and on a relatively small scale that all progress 
observed during the cOurse of selection is due to the occurrence 
step by step of fresh mutations? And how much more precarious 
would it be to affirm that these fresh mutations are all of a single 
locus, when one remembers that there are probably hundreds 
or even thousands of loci in which a mutation would have effect 
upon any given character. The evidence in the case of the 
creams is diametrically opposed to such a type of explanation, 
since the creams are manifestly of discontinuous origin in as 
many distinct loci as there are diverse modifications. The 
similarity between the creams and such a case as that of the rats 
must be attributed to the presence in the rats of the only con- 
dition met with in the creams, namely, diverse mutation. 
In the absence of rigorous tests, no one is justified in assuming 
that modifiers are not present in any given stock. Not only 
have the many selections carried out on Drosophila led experi- 
mentally to such a conclusion, but it follows from a consideration 
of the facts of mutation. In the carefully pedigreed experi- 
