Obviously the higher the intelligence and the more efficient the bodily equipment for securing food (e.g. that making possible the climbing a tree or the grasping an object), the more successful is the animal likely to be in obtaining its food supply. Thus among the rodents, the stupid rabbit, unable to climb and with little prehensile power, has to be content with a bulky diet of comparatively innutritious herbs, while the more intelligent squirrel, a nimble climber and possessed of considerable prehensile power, is able to procure highly nutritious seeds and a considerable amount of animal food as well. The intelligence and nimbleness of rats, again, enable them to procure highly concentrated and palatable foods, and to place under contribution even those that man has stored for his own use.
All the changes which the diet of our ancestors has undergone since pre-simian times are traceable to the influence of these three factors. We shall see that the food of the great apes and the monkeys is for the most part condensed, consisting of the more concentrated vegetable foods, with the addition of that most concentrated of all food - animal tissue. The fact that many of the monkeys are provided with a large caecum, and the further fact that all the great apes possess a vermiform appendix, indicating their descent from ancestors having a very capacious caecum, show that apes and monkeys are alike derived from beings whose diet was more bulky than their own. Evidently with the advance in intelligence and dexterity the evolving primate gradually abandoned the herbivorous for a more concentrated frugivorous diet, becoming also in part an animal feeder. That monkeys and apes, in spite of their great intelligence, are not more carnivorous than they are is probably to be explained by their essentially arboreal habits and their lack of the necessary bodily equipment for securing an abundance of animal food.