Every living creature has a means of survival. Plants have roots and leaves to gain nutrients and energy. Birds have wings to seek out food while escaping predators. Predators, such as lions or wolves, have speed, instincts, claws and fangs with which to chase down and consume their prey. While man cannot survive by any of these methods he does have his own means of survival- his rational faculty. His reason.
Ayn Rand sums it up this way in The Virtue of Selfishness:
Man cannot survive, as animals