BOWMAN LECTURE. LXIX 



complexities due to interaction far more intricate than any 

 yet dealt with in experimental biology. Finally the par- 

 ticulate representatives of hereditary disease must often, 

 if not always, be far less ancient in origin than those 

 representing normal charcters, and therefore presumably 

 more easily modified by disturbing influences during 

 embryonic life. 



Even the term " unit " needs to be defined, for just as 

 hardly any two persons are exactly alike even in a single 

 normal feature, and as in cases of family disease or defect 

 minor differences can often, perhaps generally, be observed 

 between the morbid appearances in one or another of the 

 affected members, so we can safely take it for granted that 

 the germinal representatives differ slightly amongst them- 

 selves in some of their attributes. The alternative would 

 be to suppose that all slight variations of inherited condi- 

 tion were due to environmental causes either before or after 

 birth. 



I propose, nevertheless, to give, for what they may be 

 considered to be worth, the numbers of affected and normal 

 actually found in the collected pedigrees of a few of the 

 diseases we are concerned with to-day, for comparison with 

 Mendelian expectation. 



Only those sibships (childships) were used that were 

 probably complete, and either contained a case or cases of 

 the disease or were the offspring of an affected parent. 

 Early deaths, stillbirths and miscarriages have been 

 omitted, as well as all sibships that were certainly, or 

 even probably, incomplete. When the disease Avas dis- 

 continuous the intervening (free) generation was not 

 counted. All these omissions, though making for 

 accuracy, entail large deductions from the total. * 



I. Acquired or post-natal cataract at all ages. Descent 

 continuous: total 440 (100), affected 177 (40), normal 

 263 (60), numbers that are quite wide of Mendelian 

 requirements. But we may assume without the least 

 hesitation that had every member been examined 

 * The data used are given in Appendix I. 



