The chief differences between the three species are the following:—
This tabular arrangement shows that Darwin's Rhea differs considerably from the other two species, whilst the latter offer apparently few important characters for separation. However, no matter if the number of the neck-vertebræ of Rh. macrorhyncha (the best name for which would be that of the long-necked or slender-headed Rhea) be a constant character or not, the differences in the proportions of the skulls afford an anatomical character just as good as those which induce us to consider Rh. darwini a so-called good species. If we thus consider the three forms of Rhea as three equivalent species, their present geographical distribution becomes less puzzling; otherwise we should expect à priori that the eastern form would differ more from the two others than these do from each other. Moreover, since it rarely happens that two large species of a certain order or family of creatures inhabit the same tracts of country unless they do so through immigration, it is probable that the original home of Rh. americana was Central South America, and that it spread from there into regions occupied by Rh. darwini.