of Heredity 5 



of single instances reveals no regularity. It is only by 

 collection of facts in great numbers, and by statistical 

 treatment of the mass, that any order or law can be 

 perceived. In the case of a chemical reaction, for instance, 

 by suitable means the conditions can be accurately repro- 

 duced, so that in every individual case we can predict 

 with certainty that the same result will occur. But with 

 heredity it is somewhat as it is in the case of the rainfall. 

 No one can say how much rain will fall to-morrow in 

 a given place, but we can predict with moderate accuracy 

 how much will fall next year, and for a period of years 

 a prediction can be made which accords very closely with 

 the truth. 



Similar predictions can from statistical data be made as 

 to the duration of life and a great variety of events, the 

 conditioning causes of which are very imperfectly under- 

 stood. It is predictions of this kind that the study of 

 heredity is beginning to make possible, and in that sense 

 laws of heredity can be perceived. 



We are as far as ever from knowing why some characters 

 are transmitted, while others are not ; nor can anyone yet 

 foretell which individual parent will transmit characters to 

 the offspring, and which will not ; nevertheless the progress 

 made is distinct. 



As yet investigations of this kind have been made in 

 only a few instances, the most notable being those of 

 Galton on human stature, and on the transmission of 

 colours in Basset hounds. In each of these cases he has 

 shown that the expectation of inheritance is such that a 

 simple arithmetical rule is. approximately followed. The 

 rule thus arrived at is that of the whole heritage of the 

 offspring the two parents together on an average contribute 

 one half, the four grandparents one-quarter, the eight 



