Meristic Variation and Reversibility of Evolution

Abstract

PROJECTING into the nasal fossa of any generalized metatherian or eutherian mammal is a series of turbinals, namely, a naso-turbinal (Fig. A, ns.t.), a maxillo-turbinal (mx.t.) and four ethmo-turbinals (I–IV), and there can be little doubt that such an arrangement characterized the remote ancestors of the Primates. But in all primitive Primates (from which we exclude the Lemures), including such forms as Tarsius (Fig. B), Hapale, Chrysothrix (Saimiri), Cebus and Lagothrix, there are but two ethmo-turbinals (I, II), for in these forms the orbital cavities have so enlarged as to obliterate by their approximation the posterior part of the ancestral nasal fossa. In the baboon, gibbon (Fig. C), chimpanzee and gorilla, and in man, the nasal fossæ are again enlarged, partly at least as a result of the growth in width of the skull-base in support of an enlarged brain, and in all these forms three¹, and in man sometimes four² or even five³, ethmo-turbinals may be developed. The phylogenetic trend in Primates seems, therefore, to have been towards a reduction of the turbinal series in early forms and a secondary expansion thereof in certain of their descendants.