Jean B. Lamarck
Though the theory of evolution's philosophical roots go back as far as Ancient Greece, it entered the agenda of the scientific world in the 19th century. In his book Zoological Philosophy, the French biologist Jean B. Lamarck hypothesized that living species had evolved from one another.
According to him, living things pass along the features they acquire during their lives, and evolve in this way. Giraffes, for example, had descended from antelope-like creatures; their necks had grown longer and longer over the generations as they sought to reach leaves from tall trees. Darwin also made use of Lamarck's thesis of the transmission of acquired characteristics as a factor that impelled evolution.
This "transmission of acquired traits" model lost all validity with the discovery of the laws of inheritance. (See The Laws of Inheritance.) With the discovery of DNA in the mid-20th century, science realized that living things possess very special genetic information encoded in the cell's nucleus, and that this information cannot be altered by behavior or striving. (See DNA.) Therefore, even if a living animal's neck did elongate by a few centimeters (an inch or two) as a result of constantly stretching up into the trees, it would still give birth to young with the standard neck measurements for its species.