Zoologists include under the Order Primates, besides man himself, the lemur-like animals (Lemuroidea) and the man-like animals (Anthropoidea). The latter include the great tailless apes (gorilla, chimpanzee, orang, and gibbon), of all animals the nearest akin to man, and the monkeys, of which the baboons are the highest representatives and the marmosets the lowest, the latter, as Huxley pointed out, being actually farther below the great apes than the great apes are below man. Nuttall has shown that, tested by the blood, the great apes are the most closely allied to man, the baboons standing next closest of kin, while the lemuroids give no indication of any blood relationship. The lemurs are the most carnivorous of the primates, while the guerezas and the langurs are the most vegetarian.
Summarizing our knowledge of the diet of the monkeys we may say that while it is in the main frugivorous it is also partly animal. Their animal food includes birds, birds' eggs, mice, lizards, frogs, shell-fish (on which certain species subsist almost entirely), grubs, scorpions, centipedes, and insects. Honey is devoured by some of the baboons and by the guenons, and gum by the baboons and probably many other monkeys. Their vegetable food comprises fruit, seeds, roots, tender shoots, and leaves. All the monkeys appear to rob plantations when they get the chance. The fact that baboons are known to dig up roots and search under stones for insects and grubs is of interest as showing the high degree of intelligence they bring to bear on the food quest, and as suggesting that man's precursors began early to use their hands in these ways. It is also worthy of note, in passing, that several varieties of monkeys employ stones and other missiles against their enemies : baboons hurl stones; guenons repel intruders with stones and branches of trees; there is even evidence that pitched battles may take place among certain species of langurs for the possession of feeding grounds - probably the chief casus belli throughout man's entire history.
Coming now to the diet of the great apes we find that, as in the case of the monkeys, they are mainly frugivorous - their vegetable diet, that is to say, consists chiefly of vegetable food in its more concentrated forms - though they are also to some extent animal feeders. The gorilla is apparently the most, and the orang the least, carnivorous of the great apes. It seems likely that animal food is chiefly sought after by them when vegetable food is scarce : insects, grubs, vermin, lizards, snakes, rats, mice and other small mammals, birds' eggs, and birds are then devoured. We have seen no mention of their consuming fish in their natural state, yet it may well be believed that they do so upon occasion. Nor have we met with any evidence of their eating honey, though seeing that this valuable and toothsome food enters into the dietary of many monkeys which are much below the apes in intelligence, it is scarcely to be doubted that the latter indulge in it when opportunity offers. Their vegetable food consists, like that of monkeys, of seeds, fruits, young shoots and leaves, succulent barks, and possibly also of roots and gums, though of this there is no direct evidence. One definite conclusion we can draw from the study of the vegetable food of the apes - namely, that since much of it is from its coarseness and acridity unfit for human consumption, the coarser and less palatable varieties must have been gradually discarded by the evolving man as the means of procuring other and better kinds increased.
To summarize then : the diet of the primates below man (i.e. of the monkeys and the great apes) is mainly vegetable, and so far as these animals are vegetable feeders they are essentially frugivorous, their food consisting, not mainly as in the case of the herbivora, of grasses, leaves, and young shoots, which in order to afford a sufficiency of nutriment must be consumed in large quantities, but of the more concentrated fruits and seeds : in short, the diet of these primates, as of all highly intelligent animals, is a concentrated diet, consisting of concentrated vegetable foods and to a less extent of the yet more highly concentrated animal foods.