Though he was building on the work of his mentor, Count George-Louis Leclerc de Buffon, Jean-Baptiste Lamarck (1744-1829) is often credited with making the first large advance toward modern evolutionary theory because he was the first to propose a mechanism by which the gradual change of species might take place. Also, he extended the definition of the change over time, saying that life started out simple and became more complex. In 1809 he published Philosophie Zoologique, in which he described a two part mechanism by which change was gradually introduced into the species and passed down through generations. His theory is alternatively referred to as the theory of transformation or simply Lamarckism. Though today Lamarck's work is considered a major step forward, in his lifetime he did not receive much recognition.
The classic example used to explain the concept of use and disuse is the elongated neck of the giraffe. According to Lamarck's theory, a given giraffe could, over a lifetime of straining to reach high branches, develop an elongated neck. A major downfall of his theory was that he could not explain how this might happen, though he discussed a "natural tendency toward perfection." Another example Lamarck used was the toes of water birds. He proposed that from years of straining their toes to swim through water, these birds gained elongated, webbed toes to better their swimming.
These two examples demonstrate how use could change a trait. By the same token, Lamarck believed that disuse would cause a trait to become reduced. The wings of penguins, for example, would be smaller than those of other birds because penguins do not use them to fly.
The second part of Lamarck's mechanism for evolution involved the inheritance of acquired traits. He believed that traits changed or acquired over an individual's lifetime could be passed down to its offspring. Giraffes that had acquired long necks would have offspring with long necks rather than the short necks their parents were born with. This type of inheritance, sometimes called Lamarckian inheritance, has since been disproved by the discovery of hereditary genetics.
An extension of Lamarck's ideas of inheritance that has stood the test of time, however, is the idea that evolutionary change takes place gradually and constantly. He studied ancient seashells and noticed that the older they were, the simpler they appeared. From this, he concluded that species started out simple and consistently moved toward complexity, or, as he termed it, closer to perfection.