There are two main senses of 'mechanism', both deriving from the metaphor of nature as a machine. One sense refers to contrivance or design, as in 'the plant's mechanism of attracting butterflies'. The other sense refers to cause or law process, as in 'the mechanism of heredity'. In his work on evolution, Charles Darwin showed that organisms are produced by a mechanism (natural selection) in the second sense, although he never used this language. He also discussed contrivance, where he did use the language of mechanism. This discussion relates metaphor in general and Darwin's use of the machine metaphor in particular to the problem of the nature of science, concluding that one use of the metaphor reinforces the objective nature of science and the other use reinforces the subjective nature of science.