  The roles that experiment plays in physics are also those it plays in biology. In the previous section we have seen that Kettlewell’s experiments both test and confirm a theory. I discussed earlier a set of crucial experiments that decided between two competing classes of theories, those that conserved parity and those that did not. In this section I will discuss an experiment that decided among three competing mechanisms for the replication of DNA, the molecule now believed to be responsible for heredity. This is another crucial experiment. It strongly supported one proposed mechanism and argued against the other two. (For details of this episode see (Holmes 2001)).