In Biological Emergences, Robert Reid argues that natural selection is not the cause of evolution. He writes that the causes of variations, which he refers to as natural experiments, are independent of natural selection; indeed, he suggests, natural selection may get in the way of evolution. Reid proposes an alternative theory to explain how emergent novelties are generated and under what conditions they can overcome the resistance of natural selection. He suggests that what causes innovative variation causes evolution, and that these phenomena are environmental as well as organismal.