"The power of habit which is found to be still so great in man, especially in one who has but slightly exercised the organ of his thought, is among animals almost insurmountable while their physical state remains the same. Nothing compels them to vary their powers, because they suffice for their wants and these require no change. Hence it is constantly the same objects which exercise their degree of intelligence, and it results that these actions are always the same in each species. "The sole acts of variation, _i.e._, the only acts which rise above the limits of habits, and which we see performed in animals whose organization allows them to, are _acts of imitation_. I only speak of actions which they perform voluntarily or freely (_actions qu'ils font de leur plein gré_). "Birds, very limited in this respect in the powers which their structure furnishes, can only perform acts of imitation with their vocal organ; this organ, by their habitual efforts to render the sounds, and to vary them, becomes in them very perfect. Thus we know that several birds (the parrot, starling, raven, jay, magpie, canary bird, etc.) imitate the sounds they hear. "The monkeys, which are, next to man, the animals by their structure having the best means to this end, are most excellent imitators, and there is no limit to the things they can mimic. "In man, infants which are still of the age when simple ideas are formed on various subjects, and who think but little, forming no complex ideas, are also very good imitators of everything which they see or hear. "But if each order of things in animals is dependent on the state of organization occurring in each of them, which is not doubted, there is no occasion for thinking that in these same animals the order which is superior to all the others in organization is proportionally so also in extent of means, invariability of actions, and consequently in intellectual powers. "For example, in the mammals which are the most highly organized, the _Quadrumana_, which form a part of them, have, besides the advantages over other mammals, a conformation in several of their organs which considerably increases their powers, which allows of a great variability in their actions, and which extends and even makes predominant their intelligence, enabling them to deal with a greater variety of objects with which to exercise their brain. It will doubtless be said: But although man may be a true mammal in his general structure, and although among the mammals the _Quadrumana_ are most nearly allied to him, this will not be denied, not only that man is strongly distinguished from the _Quadrumana_ by a great superiority of intelligence, but he is also very considerably so in several structural features which characterize him.