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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 z .

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 successive 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 themselves. I was very desirous of ascertaining the facts of the case. After much consideration and many inquiries, I determined, in 1885, on experimenting 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