  Deep and high resolution radio observations of the Hubble Deep Field and
flanking fields have shown the presence of two distant edge-darkened FR I radio
galaxies, allowing for the first time an estimate of their high redshift space
density. If it is assumed that the space density of FR I radio galaxies at z>1
is similar to that found in the local universe, then the chance of finding two
FR I radio galaxies at these high radio powers in such a small area of sky is
<1%. This suggests that these objects were significantly more abundant at z>1
than at present, effectively ruling out the possibility that FR I radio sources
undergo no cosmological evolution. We suggest that FR I and FR II radio
galaxies should not be treated as intrinsically distinct classes of objects,
but that the cosmological evolution is simply a function of radio power with FR
I and FR II radio galaxies of similar radio powers undergoing similar
cosmological evolutions. Since low power radio galaxies have mainly FR I
morphologies and high power radio galaxies have mainly FR II morphologies, this
results in a generally stronger cosmological evolution for the FR IIs than the
FR Is. We believe that additional support from the V/Vmax test for evolving and
non-evolving populations of FR IIs and FR Is respectively is irrelevant, since
this test is sensitive over very different redshift ranges for the two classes.
