Wilfrid E. Le Gros Clark (1895–1971)

The progressive differentiation and elaboration of placental mechanisms, which evidently promote a more and more efficient means of securing the optimum conditions for the developing embryo, comprise one of the most outstanding features of Primate evolution. Correlated with this, evidently, is the reduction in the number of young produced at a birth. In general, plural gestation is to be regarded as a primitive mammalian feature, and it is characteristic of almost all Primates that they give birth to only one at a time. In the tree-shrews there are normally two, one developing in each uterine horn. In the lemurs twins are not un- common in Loris, Galago and Nycticebus, but otherwise they are exceptional. In the Anthropoidea the primitive status of the Callithricidae is emphasized by the fact that this is the only group which normally produces twins (and sometimes triplets). In all other higher Primates, multiple births occur only as occasional anomalies.

We may now picture the emergence of the Primates in the Palaeocene (or more probably in the latter part of the preceding Cretaceous Period) in the form of small arboreal creatures very similar to the modern tree-shrews, alert, active and agile among the branches, relying more and more on the discriminative potentialities of the visual sense, and less and less on the more limited scope of the olfactory sense. The protection of the foliage would presumably have conferred an advantage in the struggle for existence not available to terrestrial types, and this was undoubtedly one of the main factors which permitted the development of an increasing complexity of general organization without the need for side-tracking specializations on which the terrestrial mammals had perforce to depend for their continued existence. It is the in- creasing complexity of general organization (particularly in the higher functional levels of the brain) which gives to the Primates their distinctive capacity for a wider range of adjustments to any environmental change