   The leading idea of Darwin’s theory of natural selection is often expressed in terms first coined by Herbert Spencer as the claim that among competing organisms the fittest survive (1864, 144). If there is random variation among the traits of organisms, and if some variant traits fortuitously confer advantages on the organisms that bear them, i.e., enhance their fitness, then those organisms will live to have more offspring, which in turn will bear the advantageous traits. Whence descent with adaptive modification, i.e., evolution. Evolution by random heritable variation and natural selection will explain ever increasing adaptation to given environments, increasing diversity in the occupation of new environments, and the complexity of organisms and their parts as their lineages adapt to one another and to their environments.