In my reply, this sentence from Cuvier was chosen as a motto: "Respiration is the function essential to the constitution of an animal body; it is that which, in a manner, animalizes it; and we shall see that animals exercise their peculiar functions more completely according as they enjoy greater powers of respiration."[5] My reasoning was to this effect: "It is in vain to say that cannot be, which is." When two events are so conjoined in nature that one is the only invariable antecedent of the other, then, according to all logic, we are bound to conclude that the first is the physical cause of the second, even though we cannot understand how it should be. Of the circulation, such living respiration as produces heat is the invariable antecedent, and nothing else is. The heart's action, as stated by our reviewer himself, is not therefore respiration, and not the heart's action or anything else, is the cause of the circulation. This argument is upheld by the fact that circulation, varies not only as respiration, but as its products digestion, strength, and, according to Cuvier, animal vitality vary. All begin with respiration, end with it, and are as it is. If respiration ceases, restore it before the organism is deranged, and they are all restored. We must conclude, then, that respiration is the cause of circulation, although we could not see how it should be. Much more, when we discern a mighty power, that of expansion, and see how the Almighty has made our frame in reference to its production by caloric--the lungs allowing of heat within them like wet cloth, and the nerves, bones and muscles all made and arranged, so that oxygen shall be brought to them by respiration on the one hand, and carbon by the numerous digestive and circulatory organs on the other.