72 Origin, or Process of Causation of the Series. [CHAP. in. 



At the date of Mr Galton s original paper on the subject 1 , 

 there were no available statistics of heights of human beings ; 

 so a physical element admitting of careful experiment (viz. 

 the size or weight of certain seeds) was accurately estimated. 

 From these data the actual amount of reversion from the 

 extremes, that is, of the slight pressure continually put upon 

 the extreme members with the result of crowding them back 

 towards the mean, was determined, and this was compared 

 with what theory would require in order to keep the charac 

 teristics of the species permanently fixed. Since then, 

 statistics have been obtained to a large extent which deal 

 directly with the heights of human beings. 



The general conclusion at which we arrive is that there 

 are several causes at work which are neither slight nor in 

 dependent. There is, for instance, the observed fact that the 

 extremes are as a rule not equally fertile with the means, 

 nor equally capable of resisting death and disease. Hence 

 as regards their mere numbers, there is a tendency for them 

 somewhat to thin out. Then again there is a distinct 

 positive cause in respect of reversion. Not only are the 

 offspring of the extremes less numerous, but these offspring 

 also tend to cluster about a mean which is, so to say, shifted 

 a little towards the true centre of the whole group; i.e. to 

 wards the mean offspring of the mean parents. 



18. For a full discussion of these characteristics, and 

 for a variety of most ingenious illustrations of their mode of 

 agency and of their comparative efficacy, the reader may be 

 referred to Mr Galton s original articles. For our present 

 purpose it will suffice to say that these characteristics tend 

 towards maintaining the fixity of species; and that though 

 they do not affect what may be called the general nature of 



1 Typical Laws of Heredity; read 1877. See also Journal of the An- 

 before the Royal Institution, Feb. 9, throp. Inst. Nov. 1885. 



