458 MISSES K. FOOT AND E. C. STROBELL : 



inheritance o£ the genital spot, which is published in this same volume 

 (Plates 28-34). It is only necessary to compare the two sets of photo- 

 ,«-raphs in order to determine whether any definite relation exists in the 

 inheritance o£ these two exclusively male characters. Such a comparison 

 proves conclusively that the two are not linked in inheritance (see Linkage, 



p. 475). 



The study of the genital spot in these hybrids (Foot and Strobell, 

 '13 & "14) forced certain conclusions as to its bearing on recent chromosome- 

 theories of heredity, and also its bearing on MendePs law of heredity. We 

 shall demonstrate in the following paper that the results obtained from the 

 study of the genital spot are in fact duplicated in the case of the intromittent 

 o^gan — the study of this second exclusively male character supporting in 

 every detail the conclusions forced by the facts of inheritance of the genital 

 spot. 



Results and Discussion. 

 We have dissected the intromittent organs from the genital segment of 

 many of the parent species and from all the hybrids both of the Fi and Fa 

 oenerations, and also from the offspring of the backcross. After mounting, 

 the intromittent organs were photographed at a magnification of 20 dia- 

 meters, and all the measurements were carefully made from these photo- 

 graphs ; therefore all the recorded lengths of the intromittent organs must 

 be reduced to one-twentieth to obtain the actual lengths. 



The intromittent organ of Euscliistus variolarius (at a magnification of 

 20 diameters) vnries in length between 85*5 mm. and 106 mm., while that of 

 Euschistvs servus varies between 146 mm. and 182 mm. These measurements 

 were made from 62 pure variolarius specimens, and from 62 pure servus 



specimens. 



The followino- tables give the measurements of the photographs of the 

 intromittent organs (arranged in order of length) of the parent species, of 

 hybrids, and of the backcross. 



Table 1 oives the lengths of the intromittent organs from 62 specimens of 

 Euscliistus variolarius, and shows those cases in which 2 or more specimens 

 liave the same length. 



Table 2 o-ives the lengths of the 62 intromittent organs from Euscliistus 



servus. 



Tables 3 to 11 show the lengths of the intromittent organs from all 

 the Fa hybrids and from the offspring of the backcross. These tables also 

 show to what degree the genital spot is inherited by each hybrid, and they 

 therefore demonstrate what relation, if any, exists in the inheritance of these 

 two exclusively male characters. Tables 3 to 9 inclusive give the lengths of 

 the intromittent organs for each family of the 7 pairs of Fi hybrids ; and 

 the mean leno-th of the intromittent organ is computed for each separate 

 family as well as for the total number of Fg hybrids. 



