“Giraffes acquiring long necks slowly over the years” supports

 Scientist Lamarck proposed theory  of inheritance of Acquired characters or the Theory of Use and Disuse of Organ. According to him, Use of an organ contineously led to the development and strengthening of that organ and disuse leads to weakening of that organ. He supported his theory by giving the example of "Girafees " and explained that the ancestors of Girafee were small necked with short forelimbs and were unable  to reach the leaves of upper branches.  They continuously stretched their neck to reach the vegetation.  Due to this continuous effort, they  acquired long neck and long forelimbs over the years.