According to Lamarck, the evolution of long necks in giraffes is explained by:

Explanation

In Lamarck's account each giraffe stretched to reach higher leaves, its neck lengthened a little during its own life, and that gain was passed to its offspring. Darwin's explanation instead starts from neck length varying at birth and lets the environment select among those variations, which is what the first and third options describe. This single example is the standard way of contrasting the two theories.