ABSTRACT
Darwin's references to primates in The Descent of Man (1871) were both frequent and well-scattered throughout a text devoted to evolutionary principles of significance in most animal phyla. It was however clear that of all the taxa that interested him the primates were of unique and most direct importance for human descent. In most animal groups the sexes differ not only in the character of their generative organs, their primary sexual attributes, but also in their secondary sexual characteristics, features not directly concerned with the mating act but which are highly diagnostic of the sex to which an individual belongs. Sexual dimorphism in terms of the percentage of the males' weight shown by females is most striking in the orang utan and in the gorilla (50 percent approx). The leaf eaters occur in a variety of forests and show marked intraspecific variation including both A and B grouping patterns in their social organizations.
