Another difficulty that Darwin was aware of, and it is one that is still pulled up from time to time as an objection, is the old chestnut of 'an eighth of an eye': If the raw material of natural selection consists of minute fortuitous novelties there must have been a stage at which the proto-organ had no recognizable function and would therefore have conferred no selective advantage. Therefore useful organs must have developed with a view to the function they would eventually serve. But, says Darwin, an eye might simply start out as something as simple as a light-sensitive nerve - still something of use to an organism, and the process is able to take hold from there. Moreover, an organ can be profitable in an early stage of its development, but for a different reason than the one that it eventually serves. A prime example is that of feathers, which whilst being harnessed to give the power of flight, may well first have evolved as a form of heat insulator. Evolutionary biology must always be wary of teleology - noses did not evolve to support glasses. But, as Darwin concludes, "If it could be demonstrated that any complex organ existed which could not have been formed by numerous successive, slight modifications, my theory would absolutely break down. But I can find no such case."

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