Explain the Lamarck's Theory in evolution?

Prior to Darwin, most people believed in and held to the creationist viewpoint that species were created in their current forms, which have not changed. In 1809, Jean Baptiste Lamarck proposed a new theory of evolution. He used the fossil record to explain how different adaptations arose.

Lamarck's theory incorporated the following ideas. He thought that organisms had an "inner need" to become better adapted to their environments. This, he thought, would explain the trend that organisms had toward greater and greater complexity observed in the fossil record. Secondly, Lamarck, along with others, felt that organisms are able to gradually lose characteristics that were not used over time, and further, that they could develop those characteristics that were "needed" to aid in their survival. Lastly, Lamarck thought that these newly acquired characteristics, or traits, could be passed on to future generations - this came to be known as the inheritance of acquired traits.

Today, of course, we know that there is no evidence that supports Lamarck's theory. Increasing muscle mass and strength by exercise does not alter the gene that carries that trait. This means that a runner who trains and develops running prowess is not able to pass on acquired increased muscle mass and strength to his or her offspring. Although Lamarck's Theory was ultimately replaced, Lamarck is still credited with formulating a plausible theory that was accepted by many.