sub-genera are South American and do not enter the United
States. Another genus, _Pudua_, from Chili, is much like the
brockets, but has exceedingly short cannon bones, and some of the tarsal
bones are united in a manner unlike other deer. In all, thirty specific
and sub-specific names are now carried on the roll of _Mazama_ and
its allies.

Attention has already been directed to the parallelism between the
course of progress from simple to complex antlers in the development of
the deer tribe, and the like progress in the growth of each individual,
and to the further fact that all the stages are represented in the
mature antlers of existing species. But a curious result follows from a
study of the past distribution of deer in America. At a time when the
branched stage had been already reached in North America, the isthmus of
Panama was under water; deer were then absent from South America and the
earliest forms found fossil there had antlers of the type of
_M. virginiana_. The small species with simple antlers only made
their appearance in later periods, and it follows that they are
descended from those of complex type. This third parallel series,
therefore, instead of being direct as are the other two, is reversed,
and the degeneration of the antler, which we have seen taking place in
the southern deer, has followed backward on the line of previous
advance, or, in biological language, appears to be a true case of
retrogressive evolution--representing the fossil series, as it were, in
a mirror.

The reindeer-caribou type, of the genus _Rangifer,_ agrees with
American deer in having the vertical plate of the vomer complete, and in
having the lower ends of the lateral metacarpals remaining, but, like
_Cervus,_ it has a brow-tine to the antlers. Of its early history