G ALTON AND STATISTICAL STUDY OF INHERITANCE 1 55 



sum of its ancestry, or its "mid-parentage," is, on the average, 

 nearer than any exceptional parent to the mean of the race, the 

 children of selected parents are, on the average, more mediocre 

 than their parents. 



It is quite possible that Galton's data may be valuable, and 

 that they may be trustworthy in the study of human faculties, and 

 yet that they may fail to prove this generalization ; and I shall 

 try to show that this is the case, although I am not sure I fully 

 grasp his point of view. I assume that he regards a zoological 

 type, or species, as something which owes its origin to a " principle 

 of stability " which is not itself due to selection. This is assuredly 

 the current interpretation of his statements, and it is from this 

 standpoint that I shall examine his writings. If this is not his 

 opinion ; if he in fact believes that this " principle " owes its 

 existence to past selection ; if from his data he deduces only the 

 generalization that the results of past selection may persist after 

 it has ceased to act, — I see no ground for criticism, for his data 

 assuredly prove this much, although I cannot reconcile his state- 

 ment that "the principle of stability is independent of selection" 

 with belief that it is the result of past selection. 



Before we discuss the subject it may be well to ask what evi- 

 dence there is that the child does inherit from any ancestor except 

 its parents, for descent from a long line of ancestors is not neces- 

 sarily equivalent to inheritance from them, and it is quite possible 

 that the conception of a " mid-parent " may be nothing but a 

 logical abstraction, useful, perhaps, for statistical purposes, but 

 without any real existence in nature. 



Most of its support is derived from the phenomena of rever- 

 sion or atavism ; from the appearance, in children, of ancestral 

 features which were not exhibited by the parents. While these 

 phenomena are real and familiar, we may well doubt whether 

 any of them are reversions in Galton's sense. In some cases 

 we can show that a so-called reversion is simply the manifesta- 

 tion of a possibility which is latent in the structure of all the 

 normal members of the species. The occurrence, in man, of a 

 distinct premaxillary bone is an example of this sort of rever- 

 sion. It is due to arrest of normal development, and this arrest 

 might have happened to any member of the species, with the 



