  Some species of salmon and sea turtle are famously good at finding their
birth place to reproduce after having travelled vast expanses of ocean. In
contrast, imagine now a species (maybe ancestral to the salmon or turtle) which
has to find its birth place to reproduce but has no navigation skills and
relies on chance alone. Would such an imaginary species survive? According to
our (very simple) model it would survive if and only if the probability that a
given individual find its birth place is strictly larger than 1/2.
