CROSSING TWO HEMIPTEROUS SPECIES. 227 



The results from the back cross demonstrate that the type of 

 intromittent organ distinctive of the species can (like the presence 

 or absence of the genital spot) be transmitted by both the so-called 

 male-producing and female-producing spermatozoa. The back 

 cross demonstrates not only that the male can directly transmit 

 the intromittent organ distinctive of variolarins ; but that the 

 type of intromittent organ distinctive of servus is transmitted by 

 the FI female, and therefore was transmitted by the female- 

 producing spermatozoon of the first cross. If such a primary 

 sexual character of the male can be carried by the female- 

 producing spermatozoon, it is only logical to believe that female 

 primary sexual characters also can be transmitted by both types 

 of spermatozoa (male-producing as well as female-producing) 

 for it is difficult to believe that male and female primary sexual 

 characters differ fundamentally in their method of transmission. 

 The facts appear to deprive the male-producing spermatozoon 

 of its distinctive function, and challenge the logic of endowing 

 slight morphological differences in structures of the cell with 

 causal attributes of fundamental importance. 



Linkage in Inheritance. If factors which stand for a given 

 character are carried by a definite chromosome or pair of chromo- 

 somes, and the inheritance of the character is due to a special 

 distribution of the factors at mitosis, it would seem logical to 

 expect that the factors of two characters showing a very special 

 mode of distribution (i. e., exclusively male characters) would be 

 contained in the same chromosome, and that this would be indi- 

 cated by their being linked in the hybrids. We would expect the 

 absence or presence of the genital spot, distinctive of one species, 

 to be associated in inheritance with the type of intromittent 

 organ characteristic of the same species. Even if the extent to 

 which a character appears is dependent upon hypothetical 

 factors outside the chromosomes, we would expect these hypo- 

 thetical factors to act equally on two characters which are so 

 closely associated as to be contained in the same chromosome. 

 We should expect the two characters never to be so entirely 

 dissociated that we find, in the same individual, the absence of 

 spot characteristic of one species, associated with the type of 

 intromittent organ distinctive of the other species. Instances of 



