THE STUDY OF INHERITANCE. 485 



a precise reflection of the upper. Next, let a hundred dots be 

 counted from above downward, and let a line be drawn below 

 them. According to the conditions, this line will stand at the 

 height of seventy-eight inches. Using the data afforded by these 

 two lines, it is possible by the help of the law of deviation from 

 an average to reproduce with extraordinary closeness the entire 

 system of dots on the board. 



This law of deviation from an average is not restricted to vital 

 phenomena, but holds true of all events which are the resultants 

 of variable conditions, which remain the same through all the 

 events recorded. If the marks on the board had been made by 

 bullets fired at a horizontal line stretched in front of a target, 

 they would have been distributed according to the same law, 

 their average value would be constant, and the deviations of the 

 several events from the average would be governed by the same 

 law, which is identical with that which governs runs of luck at a 

 gaming table. 



Galton has described an apparatus which mimics in a very 

 pretty way the conditions on which deviations from a mean de- 

 pend. It is a long, shallow box set on end and glazed in front, 

 leaving a depth of about a quarter of an inch behind the glass. 

 Strips are placed in the upper part to act as a funnel. Below the 

 outlet of the funnel stand a succession of rows of pins stuck 

 squarely into the backboard, and below these again are a series 

 of vertical compartments. A charge of small shot is inclosed. 

 When the frame is held topsy-turvy, all the shot runs to the 

 upper end ; then when it is turned back into its working position 

 the desired action commences. 



The shot passes through the funnel and, issuing from its nar- 

 row end, scampers deviously down through the pins in a curious 

 and interesting way : each one of them darting a step to the right 

 or left, as the case may be, every time it strikes a pin. The pins 

 are so placed that every descending shot strikes a pin in each 

 successive row. The cascade issuing from the funnel broadens as 

 it descends, and at length every shot finds itself caught in a com- 

 partment immediately after freeing itself from the last row of 

 pins. The outline of the columns of shot that accumulate in the 

 successive compartments approximates to the mathematical law 

 of frequency, and is closely of the same shape, however often the 

 experiment is repeated. 



The outlines of the columns would become more nearly identi- 

 cal with the normal law of frequency if the rows of pins were 

 much more numerous, the shot smaller, and the compartments 

 narrower ; also, if a larger quantity of shot were used. 



The principle on which the action of the apparatus depends is 

 that a number of small and independent accidents befall each 



