Chap. XII. INHERITANCE. 5 



thus appeared in a single individual out of many millions, all 

 exposed in the same country to the same general conditions 

 of life, and, again, that the same extraordinary peculiarity has 

 sometimes appeared in individuals living under widely different 

 conditions of life, we are driven to conclude that such peculia- 

 rities are not directly due to the action of the surrounding con- 

 ditions, but to unknown laws acting on the organisation or 

 constitution of the individual;— that their production stands in 

 hardly closer relation to the conditions than does life itself. 

 If this be so, and the occurrence of the same unusual character 

 in the child and parent cannot be attributed to both having 

 been exposed to the same unusual conditions, then the following 

 problem is worth consideration, as showing that the result 

 cannot be due, as some authors have supposed, to mere coin- 

 cidence, but must be consequent on the members of the same 

 family inheriting something in common in their constitution. 

 Let it be assumed that, in a large population, a particular 

 affection occurs on an average in one out of a million, so 

 that the a priori chance that an individual taken at random 

 will be so affected is only one in a million. Let the popula- 

 tion consist of sixty millions, composed, we will assume, of ten 

 million families, each containing six members. On these data, 

 Professor Stokes has calculated for me that the odds will be no 

 less than 8333 millions to 1 that in the ten million families 

 there will not be even a single family in which one parent 

 and two children will be affected by the peculiarity in ques- 

 tion. But numerous cases could be given, in which several 

 children have been affected by the same rare peculiarity with 

 one of their parents ; and in this case, more especiallv if the 

 grandchildren be included in the calculation, the odds" against 

 mere coincidence become something prodigious, almost beyond 

 enumeration. 



In some respects the evidence of inheritance is more striking 

 when we consider the reappearance of trifling peculiarities. Dr. 

 Hodgkin formerly told me of an English family in which, for many 

 generations, some members had a single lock differently coloured 

 from the rest of the hair. I knew an Irish gentleman, who, 

 on the right side of his head, had a small white lock in the 

 midst of his dark hair : he assured me that his grandmother had 



