It is commonly thought today that the theory of evolution originated from Darwin in the nineteenth century. However, the idea that species mutate over time has been around for a long time in one form or another. Therefore, by Darwin's time the idea that species change from one type into another was by no means new, but was rejected by most because the proponents of evolution could not come up with a satisfactory mechanism that would explain this change.The most influential evolutionary theories prior to Darwin were those of Lamarck and Geoffroy St. Hilaire, developed between 1794 and 1830. Lamarck suggested that species evolve through the use or disuse of particular organs. In the classic example a giraffe that stretches its neck slightly to reach higher leaves will gain in neck length, and this small gain would be passed on to its offspring. Geoffroy on the other hand suggested that the change was discontinuous, large in magnitude, and occurred at the production of offspring. However, these theories of evolution were based on prior explanations that offered no demonstrated mechanism.Darwin's theory of evolution differs in that it is based on three easily verified observations. "First, individuals within a species vary from one another in morphology, physiology, and behavior. Second, variation is in some part heritable so that variant forms have offspring that resemble them. Third, different variants leave different number of offspring". Darwin then proceeded to elaborate on the mechanism of evolution by suggesting that in the universal struggle for life, nature "selects" those individuals who are best suited (fittest) for the struggle, and these individuals in turn reproduce more than those who are less fit, thus changing the composition of the population. In addition to natural selection, Darwin also suggested that species also evolve through the complementary process of sexual selection. According to Darwin, in sexual se...
