In natural selection, favored traits are preserved and passed down since the individuals with the favored traits are more likely to mate for whatever reason.
Lamarck thought that traits gained in ones lifetime would be passed down. For instance, I am a natural blond but have died my hair brown. If the world worked according to Lamarck's theory, my offspring would also have brown hair (even though my genes do not code for brown hair). However, this is not the case.
Though the concept of genes was unknown to Darwin and Lamarck, it can be applied to both of their theories.
