EESULTS OF CROSSING TWO HEMIPTEROUS SPECIES. 483 



transmit, the spot to theii* male offspring. It seems only logical to believe 

 that the causes, whatever they are, which inhibit the spot in the females are 

 also responsible for its total or partial suppression in the Fi hybrid males, 

 but the causes suggested by our critics to account for its suppression in the 

 females obviously cannot apply to the male hybrids. Morgan ('14), after 

 admitting that the spot factors cannot be carried by the X or the Y chromo- 

 some, adds ; — " We are concerned then only with a third possibility, viz. 

 that there is something in the female condition itself that is inimical to the 

 development of the spot.'"* This something, he later explains, is the two 

 X chromosomes. He says: — "The chemical interaction between two X's 

 and the rest of the cell is of such kind that it produces a female, and the 

 -female complex, as such, is inimical to the development of a spot." 



To the defenders of the chromosome-hypotheses, this may seem a plausible 

 ex[)lanation of the suppression of the spot in the females which have these 

 two X chromosomes, but it leaves unexplained the fact that the spot in 

 the Fi males is either wholly or partly suppressed, and these males have 

 only one X chromosome, while two X chromosomes are held responsible 

 for the suppression of the spot in the females. 



The facts forced us to assume some sort of hypothetical inhibiting factors 

 for the spot, not only for the females but for the Fj male hybrids as well, 

 and this is equally necessary for the case of the intromittent organ, since it 

 is not only wholly inhibited in the females, but the length of organ strictly 

 -<;haracteristic of either of the pure species is more or less inhibited in the 

 Fj hybrids. Further, the intromittent organ is like the spot in reappearing 

 in the next generation in the length typical of the two parent species, 

 proving this to have been latent in both the females and the F^ males — its 

 full expression being inhibited by unknown factors. 



Sixth. The facts show, in the case of the genital spot, and we may now 

 add in the case of the intromittent organ as well, that if we attempt to place 

 this inhibitor in definite chromosomes, we meet with as serious difficulties as 

 those involved in assuming that the factors essential for the production of 

 the genital spot are carried by special chromosomes. In our preliminary 

 report of these experiments ('13), we discussed in full the evident results of 

 placing this inhibitor in various chromosomes — in the X chromosomes, in 

 one of the ordinary chromosomes, or in a pair of chromosomes ; and we 

 found that none of these assumptions would accord with the facts. " The 

 facts force us to regard these inhibitors as hypothetical forces which cannot 

 logically be confined to the chromosomes, and are located we know not 

 where — these hypothetical inhibitors practically doing work that has been 

 assigned to definite chromosomes." 



If the chromosome-hypotheses have a foundation in fact, it would seem 

 only logical to expect that in these insects the Y chromosome should carry 

 the factors for exclusivelv male characters, for it is the only one of the 



