  Using recent measurements of the spectrum and chemical composition of the
highest energy cosmic rays, we consider the sources of these particles. We find
that the data strongly prefers models in which the sources of the ultra-high
energy cosmic rays inject predominantly intermediate mass nuclei, with
comparatively few protons or heavy nuclei, such as iron or silicon. If the
number density of sources per comoving volume does not evolve with redshift,
the injected spectrum must be very hard ($\alpha\simeq 1$) in order to fit the
spectrum observed at Earth. Such a hard spectral index would be surprising and
difficult to accommodate theoretically. In contrast, much softer spectral
indices, consistent with the predictions of Fermi acceleration ($\alpha\simeq
2$), are favored in models with negative source evolution. With this
theoretical bias, these observations thus favor models in which the sources of
the highest energy cosmic rays are preferentially located within the
low-redshift universe.
