In this metaphor, fitness is a landscape, where useful traits are peaks and harmful ones are valleys. Wheels may be a very useful peak, but the valley around that peak is too low or too wide for the gene pool (the set of genes in the population) to move to it by genetic drift or natural selection. Stephen Jay Gould points out that evolution can only build with parts that are already available. He writes that, "wheels work well, but animals are debarred [(prevented)] from building them by structural constraints [(limits)] inherited as an evolutionary legacy."