AN ANALYSIS OF THE EFFECT OF SELECTION. 33 
female to a 6-bristled Dichet male, both parents being from the cross- 
bred plus selection series. 
It follows from the data presented above that Extended is an allelo- 
morph of Dichet intermediate between Dicheet and its normal allelo- 
morph in its somatic effect, and that it arose in a fly heterozygous for 
these two factors. It is, then, the kind of thing one would expect 
contamination of allelomorphs to produce. On the other hand, it 
seems at least equally possible to suppose that it arose as a mutation 
of one or the other allelomorph, without the presence of the other or 
the one having had any influence on the event. In any case, the 
process must be an extremely rare one, for it has been detected only 
once, in spite of the very large number of offspring of heterozygous 
Dichet flies that have been observed and bred. 
Since the Extended flies have more bristles than Dichets, it may be 
supposed that the fact that the former arose in a plus-selected series 
is significant. Such a supposition has actually been made by Castle 
(Castle and Phillips, 1914, etc.) with regard to a similar case in hooded 
rats. As has been pointed out by MacDowell (1916), a mutation in 
the direction in which selection is being made has a very much better 
chance of being discovered than has one in the opposite direction. 
Moreover, these mutations have been demonstrated only in an ex- 
tremely small number of cases; and a very elementary knowledge of 
the theory of probability will suffice to convince one that a considerable 
number of cases must be established before one can conclude that muta- 
tions are more likely to occur in one direction than in another. No 
argument based on one or two cases, however well established those 
cases may be, can carry any conviction. 
“DICHATE INTERMEDIATE.” 
The Star Dichet stock in the Columbia laboratory was found to 
have in it some flies that were indistinguishable from Extended. It 
seemed possible that these flies were due to an independent occurrence 
of the Extended mutation. Since the Star Dichet stock is kept by 
mating (Star) Dichet flies together in each generation, the mutation 
responsible for these ‘‘intermediates’’ must either have occurred in a 
Dichet fly (as did the Extended mutation), or have been in the stock 
since it was made up. The fact that Dichets are mated together in 
continuing the stock seemed, however, to show that the character 
was not true Extended, since, as we have seen above, Dichet-Extended 
flies always die. But the possibility remained that “intermediate” was 
another non-lethal allelomorph of Dichet. Accordingly, tests were 
made as follows: 
Matings of Dichet by Dichet gave some intermediates, showing 
that the continuance of the character in the stock was not dependent 
on the use of non-virgin females, and proving that the character was 
not Extended. 
