 Darwin was troubled by not having a proper understanding of the inheritance mechanism, for it left him unable to rebut a powerful objection to his theory. For a population to evolve by natural selection, the members of the population must vary—if all organisms are identical, no selection can occur. So for selection to gradually modify a population over a long period of time, in the manner suggested by Darwin, a continual supply of variation is needed. Fleeming Jenkin argued that the available variation would be used up too fast (Jenkin 1867). His reasoning assumed a ‘blending’ theory of inheritance, i.e., that an offspring’s phenotypic traits are a ‘blend’ of those of its parents. (So for example, if a short and a tall organism mate, the height of the offspring will be intermediate between the two.) Jenkin argued that given blending inheritance, a sexually reproducing population will become phenotypically homogenous in just a few generations, far shorter than the number of generations needed for natural selection to produce complex adaptations.