474 MISSES K. FOOT AND E. C. STKOBELL : 



as the genital spot would seem to justify a good deal of scepticism of the- 

 sex-determination theory which is based on the assumption of male- and 

 female-producing spermatozoa. This scepticism is greatly strengthened by 

 the further evidence that these so-called male- and female-producing sperma- 

 tozoa do not differ functionally in the transmission of such an exclusively 

 male sexual character as the intromitfcent organ itself. Further, it seems only 

 logical to believe, if exclusively male sexual chai'acters are transmitted by 

 both male- and female-producing spermatozoa, the same must be true also 

 for the exclusively female sexual characters, for it is difficult to believe that 

 the two sexes can have such fundamentally different modes of transmission. 



If we measure the amount of the variolarius inheritance in this backcross,, 

 we find an astonishing agreement between the theoretical expectation and 

 the actual result. The relative amount of variolarius to servus in the 

 offspring of tliis backcross is 3 to 1, and we should expect, therefore,, 

 the mean length of the intromittent organ of servus to be reduced by 

 75 per cent, of the difference between the mean lengths in variolarius 

 and servus. This difference is 69*71 mm., 75 per cent, of this being 

 52"28 mm. Deducting this from the mean length of the organ of servus 

 (166'41 mm.) would leave 114'13 mm. as the mean length of the organ of 

 the offspring from this backcross. The mean length is in fact 113"47 mm., 

 this being only 0"66 mm. less than the calculated expectation. 



These results lose much of their significance in view of the fact that only 

 eighteen males were secured from this backcross, but the results are almost 

 exactly repeated by a backcross with E. ictericus, in which 70 instead of 

 18 males were raised *. 



The above method of computing the mean length of organ to be expected 

 in the offspring from the two species, by a simple measure of the relative 

 part each species has contributed in the crossings, is of interest only because 

 the calculated results seem to fit the facts, but it certainly can have no 

 bearing on cases that show the Mendelian type of inheritance, nor where 

 simple unit characters are involved. Neither does it apply to the Fg genera- 

 tion, for in each of the seven families the mean length is below an exact 

 intermediate. 



Table 11 demonstrates that the intromittent organs from the offspring of 

 this backcross fail to show a typical Mendelian ratio, and as this is true also 

 in the case of the genital spot, it is a further proof of the complete agreement 

 in the results obtained from these two exclusively male characters. 



* The mean length of the intromittent organ of the offspring from the vnriolarius-ictericus 

 cross is also only a fraction of a millimetre less than the theoretical expectation, bnt in this 

 cross the slightly stronger inheritance is from the original male parent, while in tlie 

 variolarius-servus cross it is from the original female parent. In both crosses, however, the 

 stronger inheritance is slightly on the side of the shorter type of intromittent organ. Our 

 results from the variolarius-ictericus cross will be published shortly. 



