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http://dx.doi.org/10.1037/12234-006
In reducing any body of facts to a science, it is first necessary to determine the underlying units out of which the facts are made up. Chemistry was alchemy until the chemical elements were identified and isolated. Histology was terra obscura until the cell theory brought forward "cells" as the units of tissues. In the same way there could be no science of genetics until the conception was developed that the individual is a bundle of unit characters rather than a unit in itself. So it has come about that we now speak of inheritance as applied to unit characters rather than to individuals as a whole. Incidentally the fact that an organism is a combination of many units makes it easy to account for the wide diversity of forms found in nature, since the addition of a single unit greatly increases the possible combinations in successive generations. (PsycINFO Database Record (c) 2016 APA, all rights reserved)