X] Summary of Evidence as to Sex 191 



be through some extraordinary coincidence that the facts 

 fall into a system which can be represented by this sym- 

 bolism, but the convergence of the several sets of facts 

 strongly supports the belief that the symbolical system gives 

 a true representation of the physiological basis of sex in 

 those forms at least to which the system applies. 



The evidence from the descent of the dominant sex- 

 limited diseases, such as colour-blindness, and of the horns 

 in Sheep is also consistent with the same view, namely that 

 femaleness is due to the presence of a dominant factor. For 

 in these examples there is evidently some additional element 

 present in the female which inhibits or suppresses the opera- 

 tions of the sex-limited dominant, and that additional element 

 may not improbably be the factor for femaleness. 



At the same time w^e should be wrong in supposing that 

 the recognition of femaleness as a definite, allelomorphic 

 factor is more than a first step towards an understanding of 

 the phenomenon of sexual dimorphism. There are abundant 

 signs that this representation expresses only a part of the 

 truth. There seems to be no reason, a priori, why the 

 gametic constitution of the two sexes should be the same in 

 all types. In the various forms on which our conclusion is 

 founded, the arrangement, we have seen, is probably for the 

 females DR, and for the males RR. In the Insects studied by 

 E. B. Wilson and Morgan the cytological evidence suggests 

 DD for the females and DR for the males. All that can be 

 claimed is that Mendelian analysis provides a hint of the 

 way in which we may proceed in the attempt to unravel 

 these intricacies. 



It is, I think, on the lines indicated in the last paragraph 

 that we must look for a reconciliation between the cytological 

 and the experimental evidence. On the one hand the 

 cytologists show that in most orders of Insects proof that 

 the male is heterozygous can be obtained. From breeding 

 experiments we find that in Vertebrates and in one Lepi- 

 dopteran xh^ female must almost certainly be regarded as 

 heterozygous. The cytological evidence shows extraordinary 

 differences even in nearly allied forms, some having a 

 distinct unpaired chromosome, while in others this body is 

 either fully or imperfectly paired. In Lepidoptera, as it 

 happens, the accessory chromosome has not been found. 



