How would Darwin explain the development of the extremely long neck of the giraffe?

If you look at the various antelopes and other herbivores in Africa you will see that there is a variety of neck lengths with the longest being the neck of the giraffe. Animals with longer necks in the normal population would be able to feed higher in trees than others. They would also have a better view of predators. Normally this would not matter but if cirstances favoured the longer necked animals, they would thrive and breed. Over time, natural selection would favour the longer necked animals and giraffes would have appeared.