Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck, was one of the first naturalists to propose a comprehensive theory of evolution. His proposed mechanisms were different from those of Charles Darwin and Alfred Wallace, and on testing they were found to be inadequate. Though Lamarck did not hit on the idea of natural selection, his proposed theory is known to have been influential in Darwin's and Wallace's work.
Lamarck proposed two general principles of evolution: the inheritance of acquired characteristics and orthogenesis. Orthogenesis, which has been discredited by modern evolutionists, is the idea that evolution necessarily takes on a momentum of its own as it drives organisms to ever-higher levels of complexity. The modern understanding of biology has discarded this notion as theoretically unsound, noting that early organisms could hardly have gotten less complex than they already were. The apparent drive toward increased complexity is therefore largely an artifact of other factors in evolution.
The inheritance of acquired traits was Lamarck's signature proposal. If true, it would mean that species evolve by the mechanism of individuals straining to develop a single trait, such as a longer neck or stronger muscles, and then passing along that enhanced trait to offspring via some unknown mechanism. Darwin's articulation of natural selection provided an alternative explanation for diversity, that of nonrandom survival, and has been found to be much more accurate than Lamarck's work.