deciduous antler was independently developed in each of the two
continents, and while it is quite probable that approximately similar
structures have at times arisen independently, it is not easy to believe
that an arrangement so minutely identical in form and function can have
been twice evolved. On the second supposition, we have to face the fact
that there is very little evidence from palaeontology of the former
presence of the American type in Eurasia. But, on the whole, the latter
hypothesis presents fewer difficulties and is probably the correct one;
in which case two migrations must have taken place, an earlier one of
the generalized type to which _Blastomeryx_ and _Cosoryx_ belonged,
and a later one of the direct ancestor of _Mazama_. There is
little difficulty in the assumption of these repeated migrations,
for evidence exists that during a great part of the last half
of the Tertiary this continent was connected by land to the
northwest with Asia, and to the northeast, through Greenland and
Iceland, with western Europe.

The distinction between the two groups is well marked. All the
_Mazama_ type are without a true brow-tine to the antlers; the
lower ends of the lateral metacarpals only remain; the vertical plate of
the vomer extends downward and completely separates the hind part of the
nasal chamber into two compartments; and with hardly an exception they
have a large gland on the inside of the tarsus, or heel. The complete
development of these characters is exhibited in northern species, and it
has been beautifully shown that as we go southward there is a strong
tendency to diminished size; toward smaller antlers and reduction in the
number of tines; to smaller size, and finally complete loss of the
metatarsal gland on the outside of the hind leg; and to the assumption
of a uniform color throughout the year, instead of a seasonal change.