  The recent observations of the positron fraction in cosmic rays by PAMELA
indicate that the fraction of positrons to the total electronic component in
cosmic rays initially decreases in the energy region 1-10 GeV and increases
thereafter. In this paper, we show that it is natural to expect such an
increase of the positron fraction within the context of cosmic ray propagation
models. It is shown that this ratio should reach an asymptotic value of ~0.6 at
very high energies. The specific measurements by PAMELA help us to distinguish
amongst various models for cosmic ray propagation, and in particular, they
support the nested leaky box model. They also provide, in conjunction with the
observations of the total electronic component by HESS, FERMI, ATIC, and other
experiments, a way of estimating the spectrum of electrons directly accelerated
by discrete sources of cosmic rays in the Galaxy.
