300 MEMORIES OF MY LIFE 



The deviation at the 25th is i, that at the 75th 

 is +i ; so the difference between them is 2, and the 

 half difference is i . 



As these lines are being written, the circumstances 

 under which I first clearly grasped the important 

 generalisation that the laws of Heredity were solely 

 concerned with deviations expressed in statistical 

 units, are vividly recalled to my memory. It was in 

 the grounds of Naworth Castle, where an invitation 

 had been given to ramble freely. A temporary 

 shower drove me to seek refuge in a reddish recess 

 in the rock by the side of the pathway. There the 

 idea flashed across me, and I forgot everything else 

 for a moment in my great delight. 



The following question had been much in my 

 mind. How is it possible for a population to remain 

 alike in its features, as a whole, during many suc- 

 cessive generations, if the average produce of each 

 couple resemble their parents ? Their children are 

 not alike, but vary : therefore some would be taller, 

 some shorter than their average height ; so among 

 the issue of a gigantic couple there would be usually 

 some children more gigantic still. Conversely as to 

 very small couples. But from what I could thus far 

 find, parents had issue less exceptional than them- 

 selves. I was very desirous of ascertaining the 

 facts of the case. After much consideration and 

 many inquiries, I determined, in 1885, on experi- 

 menting with sweet peas, which were suggested to 

 me both by Sir Joseph Hooker and by Mr. Darwin. 

 Their merits are threefold. They have so little 

 tendency to become cross-fertilised that seedsmen 

 do not hesitate to grow differently coloured plants in 



