 Today, many of you will have heard of evolution and natural selection, but it hasn't always been this way. In the early 19th century, scientists were still very much in the dark as to how living things evolved. While the idea of evolution had been around for a while, the phenomenon went unnamed and was without a proposed mechanism until Jean-Baptiste Lamarck had an idea. Lamarck, a zoologist, noted that animals seemed very well suited to their environments and suspected that throughout their lifetimes, animals improved themselves. For example, a giraffe's long neck was thought to be the result of the animal's continual stretching in order to reach food that was higher and higher from the ground. Lamarck also thought that these changes caused by the animal's struggle to survive were passed on to the next generation which then continued the evolutionary process. In other words, a new baby giraffe would have a longer neck as a result of its parents stretching for their food. For looking into another theory of evolution, a little test for you. According to Lamarck's idea, what would happen if two animals that had spent their lives using their right limbs far more than their left limbs had offspring? If you said something similar to their offspring would have had an over-developed set of right limbs, you're right. Around two years after Lamarck's death, a young Charles Darwin set off on a voyage that would change his life and science forever, prior to setting sail on the HMS Beagle. Darwin had spent some time studying under some of Lamarck's contemporaries and thus had an understanding of Lamarck's theories on inheritance. What he found on his voyage, however, did not seem to fit with Lamarck's ideas. Instead, Darwin developed his own idea of descent with modification. The key difference here is that in Darwin's idea, the traits of the parents were passed on without being modified by the life of the parent. What Darwin realised was the importance of the fact in any population of living organisms there already exists variation in any given characteristic. As a result, he argued, nature would favour, or select, the fittest variants. This is natural selection. By fitness, Darwin meant the ability of an individual to survive and to pass on its characteristics to the next generation. Over time, changes in the characteristics of a species could occur. He also proposed that the changes within a species occurred very slowly through a gradual process. In the case of the giraffe neck, natural selection would continually favour giraffes with longer necks, so that over time the average neck length in the population would gradually increase. Following over 20 years of work, Darwin eventually published his idea of evolution by natural selection. After another man named Alfred Russell Wallace wrote him a letter saying that he had thought of the exact same thing, but that's a story for another video.