The Making of Species 



in certain species, so in some cases are certain 

 characters correlated with sex. 



Why this should be so we are not in a position 

 to say ; this, however, does not affect the indis- 

 putable fact that such correlation does exist. 



Physicians in the course of their practice 

 sometimes come across very curious cases of 

 correlation in human beings. 



" It is," writes Thomson (Heredity, p. 290), 

 " an interesting fact that an abnormal element in 

 the inheritance may find expression in the males 

 only or in the females only. If we could under- 

 stand this we should be nearer understanding 

 what sex really means. 



" Haemophilia, or a tendency to bleeding, is 

 a heritable abnormality, partly associated with 

 weakness in the blood-vessels, which do not 

 contract as they should and are apt to break, 

 and partly connected with a lack of coagulating 

 power in the blood. It is usually confined to 

 males. But as it passes from a father through a 

 daughter to a grandson, and so on, it must be a 

 latent part of the germinal inheritance of the 

 females, though for some obscure physiological 

 reason it fails to find expression in them, or has 

 its expression quite disguised. Colour-blindness 

 or Daltonism has been recorded (Horner) through 

 the males only of seven generations. Dejerine 

 cites another case (fide Appenzeller) in which all 

 the males in a family history had cataract through 



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