CASTLE: THE HEREDITY OF SEX. 213 



are calculated the numbers inserted in the last four columns of Table IV., 

 resfard beiner 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 Aeries) result from resolution 

 of these couplings 1 



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. 



