290 HEREDITY AND DISEASE 



amphibians, fishes, and lower animals, there is some evidence 

 that analogous factors may occasionally operate in mammals. 

 Thus, the pressure of amniotic strands may divide the rudiment 

 of a limb into two or may cause a mutilation. All such cases 

 are equivalent to accidents in after-life ; they are in no way ex- 

 pressions of the inheritance, and there is no evidence to show 

 that they have any effect upon the inheritance. 



" The Hapsburg lower lip or the large nose of Orleans is truly 

 an item in the inheritance, but the occasional absence of an arm 

 (due to a constriction of the rudiment by a strand of the amnion) 

 is an intra-uterine acquisition ; it is congenital, but it is not 

 inherited " (Martius, 1905, p. 14). 



It has sometimes been remarked that certain families show 

 a hereditary tendency to have wens (" small cystic or encysted 

 tumours ") on the head and upper parts of the body. The nature 

 of the growth, its inconstant position, and the time at which it 

 appears (usually about middle age) show that we should not 

 speak of the inheritance of a wen, but rather of the inheritance 

 of some skin-weakness. 



§ 8. Some Provisional Propositions 



I. Abnormal Peculiarities may find Expression in One Sex 

 only. — (a) Most of man's defects and predispositions to disease are 

 transmitted to both sexes (equally or unequally) through suc- 

 cessive generations. Polydactylism and some forms of cataract, 

 Huntingdon's chorea and diabetes insipidus, may be given as 

 instances. The liability of the sexes is in some cases very 

 unequal ; thus exophthalmic goitre is rare in males. 



(b) In some other cases, such as albinism, there is a marked 

 tendency to skip a generation or even two generations, but as in 

 the first group both sexes are liable to be affected. Albinism is 

 Well known to be a recessive character, and we can readily under- 

 stand, as Mott points out, how the marriage of two apparently 



