Whatever number of inches or decimals of an inch the species
varies in any of its parts is marked on the diagrams, so that with the
help of an ordinary divided rule or a pair of compasses the variation of
the different parts can be ascertained and compared just as if the
specimens themselves were before the reader, but with much greater ease.
In my lectures on the Darwinian theory in America and in this country I
used diagrams constructed on a different plan, equally illustrating the
large amount of independent variability, but less simple and less
intelligible. The present method is a modification of that used by Mr.
Francis Galton in his researches on the theory of variability, the upper
line (showing the variability of the body) in Diagrams 4, 5, 6, and 13,
being laid down on the method he has used in his experiments with
sweet-peas and in pedigree moth-breeding.[25] I believe, after much
consideration, and many tedious experiments in diagram-making, that no
better method can be adopted for bringing before the eye, both the
amount and the peculiar features of individual variability.
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