   The diversity, complexity, and adaptation of the biological realm is evident. Until Darwin, the best explanation for these three features of the biological was the conclusion of the “argument from design.” Darwin’s theory of natural selection provides an explanation of all three of these features of the biological realm without adverting to some mysterious designing entity, by explaining the process of “the survival of the fittest.” But this explanation’s success turns on the meaning of its central explanatory concept, ‘fitness’. Moreover, since Darwinian theory provides the resources for a purely causal account of teleology, wherever it is manifested, its reliance on the concept of ‘fitness’ makes it imperative that conceptual problems threatening the explanatory legitimacy of this notion be solved.