AN ANALYSIS OF THE EFFECT OF SELECTION. 33 



female to a 6-bristled Dichset 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 Dichset intermediate between Dichset 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 

 Dichset flies that have been observed and bred. 



Since the Extended flies have more bristles than Dichsets, it may be 

 supposed that the fact that the former arose hi 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 hi 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. 



"DICH/ETE INTERMEDIATE." 



The Star Dichset 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 Dichset stock is kept by 

 mating (Star) Dichset flies together in each generation, the mutation 

 responsible for these "intermediates" must either have occurred hi a 

 Dichset fly (as did the Extended mutation), or have been in the stock 

 since it was made up. The fact that Dichsets are mated together in 

 continuing the stock seemed, however, to show that the character 

 was not true Extended, since, as we have seen above, Dicheet-Extended 

 flies always die. But the possibility remained that ' f intermediate ' ' was 

 another non-lethal allelomorph of Dichset. Accordingly, tests were 

 made as follows : 



Matings of Dichset by Dichset 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. 



