Evolution is defined as change in the allele frequency within a gene pool that can lead to changes in an organism’s morphology (form and structure) over time. Evolution involves the processes of mutation, natural selection, and speciation, which will be introduced in upcoming sections. Prior to the 19th century, the prevailing idea in Western thought was that nature was fixed and static; it was made by a supreme being in the form it currently appeared, and it did not change. Within this fixed natural system, living creatures were arranged within a set order that was considered to have been decreed by God, known as the great chain of being. This order featured God at the top, angels beneath God, and then humans. Below humans were various types of animals, followed by plants and minerals. This hierarchy was significant both because it placed some creatures above others and because it distinctly separated humans from the rest of the animal world. During a period stretching from the 14th through the 18th centuries, some people began to question whether the natural world was as static as it was traditionally perceived to be. The British scientist and architect Robert Hooke is remembered as the first person in the Western world to claim not only that nature has changed over time but also that evidence of these changes remain. He hypothesized that fossils are the remains of actual plants and animals that were once alive. This conclusion was contrary to the previously accepted conclusion that fossils were nothing more than stone images. Hooke also noted that many marine fossils were located far away from any existing ocean, and he came to the then radical conclusion that Earth’s geography and physical features had experienced dramatic changes. The first person to propose a mechanism by which species could change was French naturalist Jean-Baptiste Lamarck, best known for having developed the first theory of macroevolution, a hypothesis about how the actual transformation from one species into another species could occur. Lamarck’s theory relied on the now defunct idea of the inheritance of acquired characteristics. Lamarck argued that the usefulness of a trait or organ could be ascertained based on its complexity or size. In particular, he believed that the usefulness of an organ could be judged by its size and the usefulness of a trait by its complexity. He speculated that organs and traits that help a creature to survive will become bigger and more complex over time, while those that are of little use will become smaller and simpler and eventually disappear. His classic example of this theory in action is the long neck of a giraffe. Lamarck speculated that as giraffes stretched their necks to reach the leaves at the tops of trees, their necks would grow longer, and furthermore, these longer necks would be inherited by the subsequent generations. This theory of the inheritance of acquired characteristics is also known as Lamarckian inheritance. One of the interesting things about Lamarck’s theory is that he believed that wishes, desires, wills, and needs were all sufficient to motivate change. That is, wishing for or desiring a change in one’s physical characteristics could make that change happen. There are two primary problems with Lamarckian inheritance. First, desires, wishes, and needs do not change physical characteristics without a deliberate change in behavior. Someone may wish for blue hair, but their hair color will not change without dye. The second problem is that the inheritance of acquired traits is not possible. If someone dyed their hair blue, their children would not inherit blue hair. Traits that are acquired during a lifespan are not passed on to subsequent generations. Just because Lamarck’s theory of macroevolution is not correct does not mean that it is insignificant. Lamarck recognized the importance of interactions between organisms and their environments in the evolutionary process and was the first to propose a mechanism by which evolutionary change from one species into another could actually occur. Georges Cuvier, another Frenchman and a leading scientist in the early 19th century, made numerous contributions to evolutionary thinking. He is best known for his theory of catastrophism, which he developed to explain the increasing number of fossils that were being found, some displaying impressions of creatures no longer found anywhere on Earth. Catastrophism proposes that floods, earthquakes, and other natural disasters—understood within the theory as acts of God—have been responsible for killing all the animals alive in certain places at certain times. According to Cuvier, either new animals have been created or the areas had been repopulated by animals from neighboring areas. To be consistent with emerging fossil evidence indicating that organisms had become more complex over time, Cuvier proposed that new organisms with a more modern appearance were the result of a more recent creation event. While scientists no longer adhere to catastrophism as a viable theory, Cuvier’s idea of extinction continues to be an important component of evolutionary thinking today. Another major contributor to evolutionary thinking was Scottish geologist Charles Lyell, known as the father of modern geology. He wrote a three-volume treatise, Principles of Geology (1830–1833), in which he argued that contemporary geological processes were the same as those that occurred in the past. These processes, such as wind and rain, produced the contemporary geological landscape. Mountains, lakes, and rivers were all created by these geological processes, many of them slow moving. This theory has come to be known as the principle of uniformitarianism. Lyell suggested that in order for such slow-acting forces to produce momentous change, Earth must be much older than previously suspected. Prior to Lyell’s publication, the majority of natural historians believed that the earth was less than 6,000 years old, a number arrived at through calculations made based on the Old Testament. By altering the suspected age of the earth from several thousand years to millions of years, Lyell changed the framework within which scientists viewed the geological past.