CROSSING EUSCHISTTTS VARIOLARIUS AND EUSCHISTUS SBRVUS. 361 



We cannot logically explain the facts on the assumption of variation in 

 potency of a single spot factor, for on this assumption the spot of pure 

 variolarius should show the same relative amount of variation, which is not 

 the case. But if we assume that servus has an inhibitor which also varies in 

 potency, then the Fj spot, besides being more or less reduced by a single 

 dose of spot factor which varies in potency, can be further reduced or wholly 

 cancelled by the servus inhibitor, according to the degree of its potency. 



The hypothesis of varying potency of unit factors demands the presence 

 of at least a perceptible variation in the spot of pure variolarius, and such a 

 variability does, in fact, occur. This irregularity can be best appreciated by 

 comparing the male offspring from the same parents. 



Castle (1912) says : " In my experience ever^ unit character is subject to 

 quantitative variation, that is, its expression in the body varies." The funda- 

 mental explanation of his striking results in selection is the assumption 

 that these variations have a germinal basis and are inherited. 



It seems to us that our results are more in harmony with Castle's assump- 

 tion of the varying- potency of unit factors ; but if they do in fact admit 

 of a pure Mendelian interpretation, this must be left to the specialist in 

 genetics. 



APPENDIX I. 



Since this paper was finished, striking corroborative support has been 

 given to the results of our experiments. 



In expressing to Professor Poulton our desire to find an experienced Ento- 

 mologist who would look over the parent species of our hybrids for some 

 distinguishing character (other than the genital spot) that might give 

 additional evidence in support of our experiments, Professor Poulton kindly 

 suggested Dr. Harry Eltringham, of New College, Oxford, We feel very 

 grateful to Professor Poulton for his interest and courtesy in this matter, 

 and we are deeply indebted to Dr. Eltringham for his very valuable discovery 

 of the marked difference in the length of the intromittent organ of E. vario- 

 larius and ]^. servus. As a result of his observation^ we have been able to 

 follow out the inheritance of this second exclusively male character in the 

 hybrids from this cross, as well as from the cross between E. variolarixis 

 and E. ictericus. 



