" The differences between Darwin and Lamarck's theories primarily stem from the second category, under content, dealing with how "organisms progress from 'lower' to 'higher' forms'" (Firenze 1997). The key difference between Lamarck's and Darwin's ideas occurs when a variation appears concerning this progression from lower to higher. Lamarck's position is that acquired characteristics arise after the environment changes. That is, a change in the environment will lead to a change in the organism. But Darwin, on the other hand, believed that variations are present in a population before the environment changes. Lamarck saw evolution in terms of individuals, but Darwin realized that populations are important. Darwin stated that random differences between individuals affect fitness, and that more fit individuals tend to survive and reproduce. In contrast, Lamarck theorized that organisms become more fit in response to their environment....

Although abandoned over 150 years ago, Jean Baptiste de Lamarck's theory of acquired characteristics - the concept that changes acquired during an organism's lifetime are somehow transferred into genetic information and passed on to the offspring - is perhaps more commonly held as the mechanism of how evolution works, than the more scientifically supported Darwinian concept of variation and selection, though both theories have had a significant impression on the scientific community.
References

Firenze, Richard 1997, 'Lamarck vs. Darwin: Dueling Theories,' Reports of the National Center for Science Education, vol.17, no.4, pp. 9-11, viewed 12 August 2005, http://64.233.161.104/search?q=cache:TcH7RBNIZrMJ:www.indiana.edu/~ensiweb/lessons/lam.dar.pdf+comparing+darwin+and+lamarck&hl=en.

Wikipedia 2005, "Darwin" and "Lamarck." Separate Searches, viewed 12 August, 2005, http://www.wikipedia.org.