80 INHERITANCE. 



Chap. XIV. 





attended to this subject, and tabulated a large number of 



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 genera- 

 tions 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." So that the longevity of a non-affected 

 parent seems to have the power of determining in the offspr 

 the fatal period ; 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 in some degree 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 oblite- 

 ration of characters proper to the embryo 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 ten- 

 dency to lose at too early an age its own proper character. 



Finally, from the numerous races of cultivated plants and 

 domestic animals, in which the seed or eggs, the young or 

 old, differ from each other and from their parent-species 

 from the cases in which new characters have appeared at a 

 particular period, and afterwards have been inherited at the I 



same period ; — and from what we know with respect to disease 

 we must believe in the truth of the great principle of inherit 

 ance at corresponding periods of life. 



> 



Summary of the three preceding Chapters. — Strong as is the 

 force of inheritance, it allows the incessant appearance of new 

 characters. These, whether beneficial or injurious, of the most 

 trifling importance, such as a shade of colour in a flower, a 

 coloured lock of hair, or a mere gesture; or of the highest 

 importance, as when affecting the brain or an organ so perfect 





