  We present a systematic examination of the changes in semi-major axis caused
by the mutual interactions of a group of massive bodies orbiting a central star
in the presence of eccentricity dissipation. For parameters relevant to the
oligarchic stage of planet formation, dynamical friction keeps the typical
eccentricities small and prevents orbit crossing. Interactions at impact
parameters greater than several Hill radii cause the protoplanets to repel each
other; if the impact parameter is instead much less than the Hill radius, the
protoplanets shift slightly in semi-major axis but remain otherwise
unperturbed. If the orbits of two or more protoplanets are separated by less
than a Hill radius, they are each pushed towards an equilibrium spacing between
their neighbors and can exist as a stable co-orbital system. In the
shear-dominated oligarchic phase of planet formation we show that the feeding
zones contain several oligarchs instead of only one. Growth of the protoplanets
in the oligarchic phase drives the disk to an equilibrium configuration that
depends on the mass ratio of protoplanets to planetesimals, $\Sigma/\sigma$.
Early in the oligarchic phase, when $\Sigma/\sigma$ is low, the spacing between
rows of co-orbital oligarchs are about 5 Hill radii wide, rather than the 10
Hill radii cited in the literature. It is likely that at the end of oligarchy
the average number of co-orbital oligarchs is greater than unity. In the outer
solar system this raises the disk mass required to form the ice giants. In the
inner solar system this lowers the mass of the final oligarchs and requires
more giant impacts than previously estimated. This result provides additional
evidence that Mars is not an untouched leftover from the oligarchic phase, but
must be composed of several oligarchs assembled through giant impacts.
