 Charles Darwin and Alfred Wallace are the two co-discoverers of natural selection (Darwin &amp; Wallace 1858), though, between the two, Darwin is the principal theorist of the notion whose most famous work on the topic is On the Origin of Species (Darwin 1859). For Darwin, natural selection is a drawn-out, complex process involving multiple interconnected causes. Natural selection requires variation in a population of organisms. For the process to work, at least some of that variation must be heritable and passed on to organisms’ descendants in some way. That variation is acted upon by the struggle for existence, a process that in effect “selects” variations conducive to the survival and reproduction of their bearers. Much like breeders choose which of their animals will reproduce and thereby create the various breeds of domesticated dogs, pigeons, and cattle, nature effectively “selects” which animals will breed and creates evolutionary change just as breeders do. Such “selection” by nature, natural selection, occurs as a result of the struggle for existence and, in the case of sexual populations, the struggle for mating opportunities. That struggle is itself the result of checks on the geometric population increase that would occur in the absence of the checks. All populations, even slow-breeding ones such as those of elephants, will increase in size in the absence of limitations on growth that are imposed by nature. These checks take different forms in different populations. Such limitations may take the form of limited food supply, limited nesting sites, predation, disease, harsh climactic conditions, and much else besides. One way or another, only some of the candidate reproducers in natural populations actually do reproduce, often because others simply die before maturity. Owing to the variations among the candidate reproducers, some have better chances of making it into the sample of actual reproducers than do others. If such variations are heritable, the offspring of those with the “beneficial” traits will be likely to produce especially many further descendants themselves. To use one of Darwin’s own examples, wolves with especially long legs that allow them to run more quickly will be more likely to catch prey and thereby avoid starvation and so produce offspring that have especially long legs that allow them, in turn, to breed and produce still more long-legged descendants, and so on. By means of this iterative process, a trait conducive to reproduction that is initially found in one or a few population members will spread through the population.