CASTLE: THE HEREDITY OF SEX. 213 
are calculated the numbers inserted in the last four columns of Table IV., 
regard being had for the observed ratio of males to females in each 
cross. Thus the males in each cross between hybrid parents are dis- 
tributed between D and R in the ratio, 8 : 1; and the females in the 
ratio, 6 : 3. 
To sum up, an examination of Table IV. shows in three of the six 
crosses considerable discrepancies between the calculated Mendelian 
ratios of D to R and those actually observed. In two of the three 
crosses mentioned, the discrepancies are satisfactorily accounted for on 
the assumption that coupling occurs in about one out of three cases 
among the gametes produced by hybrids, on the one hand between the 
male sex-character and the aberrant form-character, and on the other 
hand between the female sex-character and the species form-character. 
The same assumption explains satisfactorily the peculiar sexual distribu- 
tion of dominant and recessive forms in all five broods, if we suppose 
further that coupled gametes are sexually dominant over uncoupled 
ones, and the gametes of hybrids over those of recessive individuals, 
The principles of coupling involved in this case may serve to explain 
other apparent exceptions to Mendel’s law. We have seen how devia- 
tions from the expected ratios of dominants to recessives may result 
from partial coupling of each with a different sex-character. Complete 
coupling of this sort must necessarily result in the production of a 
stable or self-perpetuating hybrid form. In case the hybrid form is indis- 
tinguishable from a pure dominant, its real nature may be unsuspected, 
until a cross with a third form may serve to break the coupling and 
bring to light a series of new combinations. How many of our suppos- 
edly pure species may be sexually coupled hybrids? May it not be that 
many aberrant variations (mutations, de Vries) result from resolution 
of these couplings ? 
Furthermore, the principle of coupling affords an explanation of the 
inheritance of sexual dimorphism in general. There is one set of form- 
characters coupled with the male sex-character, another with the female. 
Dominance in the zygote of one sex-character necessitates dominance 
also of the form-characters which are coupled with it, while the other 
sex-character and ‘the form-characters coupled with it together become 
recessive. 
The author desires to thank Professor E. L. Mark for valuable assist- 
ance in the revision of his manuscript and proofs. 
