LXXrV BOWMAN LECTURE. 



sometimes taking place irregularly in successive siblings, 

 does not seem to occur in successive generations.* 



Two different heritable conditions may be met with 

 in the same pedigree, and care is then necessary to 

 distinguish coincidence from correlation or equivalence. 

 Thus in Fig. 1 1 we have congenital lamellar cataract, 

 congenital ptosis, and progressive goitre; and it is seen 

 that, whatever may have been the source of the ptosis, 

 the goitre undoubtedly came in from an entirely distinct 

 stock. Many analogous cases might be quoted. 



Reference has been made to cases in which a heritable 

 condition, though apparently limited to one anatomical or 

 physiological system, may invade different parts of that 

 system in different persons. The best of the well-defined 

 cases is seen in the trio retinitis pigmentosa, progressive 

 nerve deafness, and feeble-mindedness or idiocy, diseases 

 that seem capable of acting as mutual equivalents or 

 substitutes; some correlation also seems to exist between 

 Leber's disease and epilepsy; and of course the neuro- 

 pathic constitution may show itself in several different 

 forms of mental disease. Albinism is also compli- 

 cated with defects of the nervous system in a dispro- 

 portionate number of cases, and the association must 

 therefore be looked upon as more than a coincidence. 

 The possibility that early death may in certain cases 

 represent a substitute form of a heritable disease has 

 already been mentioned. 



It appears that the individuals affected by hereditary 

 imperfections and disease are very often members of unusu- 

 ally large sibships. This has been mentioned by Dr. James 

 Taylort in relation to hereditary ataxy, whilst Karl 

 Pearson J concludes that both tuberculous and deaf-mute 

 stocks are quite as fertile as, and probably more fertile 



* Darwin makes a general statement to the same effect (Animals and 

 Plants under Domestication, ii, p. 56). 



t James Taylor, T.O.S., xvii, 1897, p. 63. 



I A First Study of the Statistics of Pulmonary Tuberculosis, 1907, p. 20. 



