//A person shipwrecked on a deserted island with absolutely no chance of rescue would presumably have to start all over again, & would live without any moral values whatsoever.//
That's debatable -
http://www.thethinkingatheist.com/forum/Thread-Morality-on-a-desert-island?page=3
On a desert island you have no less need to be aware of your options to make the right choices, among them to value the continuation of your existence enough to put forth the thought and effort required to sustain and promote it. The one thing you could dispense with would be the need to establish and agree to honour a mutually beneficial bill of rights.
Valuing ones own life and understanding the means and process by which one sustains and promotes it are the foundation upon which morality is established and built, apart from which no rational moral code is either necessary or possible.