The section represented by M. virginiana, with antlers curving forward and tines projecting from its hinder border, takes practically the whole of America in its range, and under the law of variation which has been stated, has proved a veritable gold mine to the makers of names.  At present it is utterly useless to attempt to determine which of the forms described will stand the scrutiny of the future, and no more will be attempted here than to state the present gross contents of cervine literature.  The sub-genus Dorcelaphus contains all the forms of the United States; of these, the deer belonging east of the Missouri River, those from the great plains to the Pacific, those along the Rio Grande in Texas and Mexico, those of Florida, and those again of Sonora, are each rated as sub-species of virginiana; to which we must add six more, ranging from Mexico to Bolivia.  One full species, M. truei, has been described from Central America, and another rather anomalous creature (M. crookii), resembling both white-tail and mule deer, from New Mexico.

The other sub-genera are Blastoceros, with branched antlers and no metatarsal gland; Xenelaphus, smaller in size, with small, simply forked antlers and no metatarsal gland; Mazama, containing the so-called brockets, very small, with minute spike antlers, lacking the metatarsal and sometimes the tarsal gland as well.  The last three 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.