90 MODES OF RESEARCH IN GENETICS 



this way : an event A is about to happen. It 

 may happen in any one of n different ways, each 

 one of which ways may be designated by a letter, 

 I, p, r, t, etc. Now an indefinitely large number of 

 causes are concerned in bringing it about that 

 the event A is going to happen, and that it can 

 equally well happen either as Z, p, r, t, etc. In 

 other words, the setting of the stage for the event 

 has involved a vast number of small and balanced 

 causes. But the causes which are differential in 

 the particular case, that is, which determine that 

 A shall happen in the p way this particular time, 

 and not in the /, the t, or any other way, are, in 

 general : 



1. Few in number. 



2. Immediate in time. 



3. Large in relative quantitative effect. 



The point under discussion may perhaps be 

 made plainer by a homely illustration. Suppose 

 a man steps up behind a mule and prods the 

 creature with his walking stick. The human in- 

 tellect is unequal to the task of predicting exactly, 

 in the particular case, what precise portion of the 

 man's body the mule's hoof will land upon. A 

 multitude of minor causes will affect this : the 

 relative height of the man and the mule, the age 

 of each, the place poked with the walking stick, 

 the degree of fatigue of the mule, the temperature, 

 the season of the year, and countless other things 



