56 INHEEITANCE. Chap. XIV. 



on during three or even more generations in several members 

 of the same family at the same age, especially in the case of 

 rare affections in which the coincidence cannot be attributed 

 to chance, and to doubt that there is a strong tendency to in- 

 heritance in disease at corresponding periods of life. When 

 the rule fails, the disease is apt to come on earlier in the child 

 than in the parent ; the exceptions in the other direction 

 being very much rarer. Dr. I^ucas "^^ alludes to several cases 

 of inherited diseases coming on at an earlier period. I have 

 already given one striking instance with blindness during 

 three generations ; and Mr. Bowman remarks that this fre- 

 quently occurs with cataract. With cancer there seems to be 

 a peculiar liability to earlier inheritance : Sir J. Paget, who 

 has particularly attended to this subject, and tabulated a large 

 number of cases, informs me that he believes that in nine 

 cases out of ten the later generation suffers from the disease 

 at an earlier period than the previous generation. He adds, 

 " In the instances in which the opposite relation holds, and 

 the members of later generations have cancer at a later age 

 than their predecessors, I think it will be found that the 

 non-cancerous parents have lived to extreme old ages." 80 

 that the longevity of a non-affected parent seems to have the 

 power of influencing the fatal period in the offspring ; and 

 we thus apparently get another element of complexity' in 

 inheritance. 



The facts, showing that with certain diseases the period of 

 inheritance occasionally or even frequently advances, are 

 important with respect to the general descent-theory, for they 

 render it probable that the same thing would occur with 

 ordinary modifications of structure. The final result of a 

 long series of such advances would be the gradual obliteration 

 of characters proper to the embrj^o and larva, which would 

 thus come to resemble more and more closely the mature 

 parent-form. But any structure which was of service to the 

 embryo or larva would be preserved by the destruction at 

 this stage of growth of each individual which manifested 

 any tendency to lose its proper character at too early an age. 



*5 ' Hered. Xat.,' torn. ii. p. 852. 



