From Scientific American:
Giraffes’ long necks are perfectly suited to harvesting tender leaves beyond the reach of other herbivores. Pondering the genesis of this phenomenon, two giants of modern biology, Jean-Baptiste Lamarck and Charles Darwin, arrived at remarkably different hypotheses. Lamarck believed that constant stretching of the neck somehow stimulated its growth. The giraffe would then pass along this new trait to its offspring. In effect, this newer, longer neck was a direct result of a giraffe’s interaction with its environment. By contrast, Darwin’s theory posited that traits evolve as part of a random, gradual process. The giraffes that happened to have been born with longer necks thanks to a random genetic mutation were better fed and thus healthier than their shorter-necked counterparts, making them more likely to live long enough to breed and pass on this trait. Because this mutation conferred a specific advantage to long-necked giraffes that aided their survival, the trait was preserved in future generations.
Lamarckian theories about the influence of the environment were largely abandoned after scientists discovered that heritable traits are carried on the genes encoded by our DNA. A recent study, however, published by neuroscientists Junko A. Arai, Shaomin Li and colleagues at Tufts University, shows that not only does the environment an animal is reared in have marked effects on its ability to learn and remember, but also that these effects are inherited. The study suggests that we are not the mere sum of our genes: what we do can make a difference.