Dawkins argues that the best way to look at evolution is in terms of selection occurring at the lowest level possible, by which he means genes and not individuals, species or any other level. In other words, the fundamental units of natural selection are genes-molecules of DNA which he refers to as the "replicators" because they exactly recreate the properties of self. He suggests that these replicators discovered a way to protect themselves, either chemically or by building a physical wall of protein around themselves. This wall then developed into containers, vehicles for their continued existence, which Dawkins called "survival machines". These survival machines existed to enable successful gene replication and to serve as vehicles for the transportation of genes from one generation to the next, becoming bigger and more elaborate as competition for resources increased. According to Dawkins, it is the ability of the gene to design a successful "vehicle" that determines their success or failure in evolution. These survival machines have evolved into the organisms present today and therefore the theory postulates that animal behaviour is ultimately driven by the genes' selfish attempts to survive and reproduce.