Cunningham, Unisexu.il Inheritance. 7 



There is one well known abnormality in the human race which 

 appears at first sight to offer evidence of this kind. I refer to the 

 disease called haemophilia, a congenital tendency to excessive blee- 

 ding. This disease is comparatively rare, but its importance from 

 our present point of view is due to the fact that it is strongly 

 hereditary, and that it occurs chiefly in men, rarely in women. Ac- 

 cording to Wick ham Legg, one of the principal authorities on the 

 subject, the disease generally appears in the sons of women who be- 

 long to an affected family, though the women themselves show the 

 symptoms but slightly or not at all; on the other hand fathers are 

 said rarely to transmit the disease to their sons. The exact nature 

 of the abnormality to which the bleeding is due seems to be doubtful, 

 Dr. Gam gee considered the fault lay rather in the blood vessels than 

 in the blood. Weismann refers to this disease, and explains it 

 according to his theory. He suggests that the determinants of the 

 blood-vessels in the human ovum are double determinants, one set 

 developing in the male, the other in the female, and that the congenital 

 variation giving rise to haemophilia has arisen only in the male deter- 

 minants. But it is to be]observed that the disease does not exactly 

 correspond to a secondary sexual character. It generally shows itself 

 in boys during the first year of life, though sometimes the symptoms 

 do not appear until a few years later, whereas secondary sexual 

 characters generally do not appear much before the period of puberty. 

 It seems to me quite possible that the actual defect, or variation, 

 whatever it may be, is equally present in both sexes, but produces 

 more serious results in males in consequence of differences normally 

 present between the sexes. That there are normal differences between 

 man and woman in the blood, is certain, in man there are more red 

 corpuscles and the specific gravity is higher. The general blood 

 pressure also may be higher in man. Haemophilia therefore is not 

 shown to be a case of a unisexual congenital variation at all. 



Evidence to be of real importance in this enquiry should consist 

 of cases in which a unisexual congenital variation is observed to occur 

 in a species in which the male and female are normally similar. 

 Wild species of pigeons fulfil this condition, and it is a fact that sexual 

 differences have appeared to a certain degree in pigeons under domesti- 

 cation. Darwin refers to a Belgian breed in which the males alone 

 are marked with black striae, and the peculiarities of the pouter and 

 carrier are more developed in the male than in the female. But it is 

 by no means certain that these differences arose as blastogenic 

 variations: it seems to me more probable that they are to be explained 

 in the same way as I explain the sexual differences in wild animals. 

 The occurrence of unisexual variations in individual pigeons has not I 

 believe been described. 



