Taking the heart then for the centre of the animal machine, we find in that and other adjacent parts, there is a perfect resemblance between man and other animals: but the more we remove from this centre, the more they become different; and when in the centre itself there is found any difference, then the animal is infinitely more distant from man, and possesses nothing in common with those animals we are now considering. In most insects, for example, there is a peculiar organization of this principal part of the animal economy. Instead of heart and lungs, they have parts which, being subservient to the vital functions, have been considered as analogous to those viscera, but which in reality widely differ from them, both in structure and result of action, and therefore are insects to the last degree different from man and other animals. A minute difference in the centrical parts is always accompanied with an infinitely greater in the exterior parts. The tortoise, whose heart is of a peculiar structure, is a very extraordinary animal, and has not the smallest resemblance to any other animated being. In considering men, quadrupeds, birds, cetaceous animals, fishes, reptiles, &c. what prodigious variety do we find in the figure and proportion of their bodies, in the number and position of their members, in the substance of their flesh and bones? Quadrupeds have generally tails and horns; cetaceous animals live in another element, and though their mode of generation is similar to that of quadrupeds, yet they differ greatly from them in form, having no inferior extremities; birds differ still more by their beaks, feathers, wings, and their propagation by eggs; fishes and amphibious animals are yet farther removed from the human form, and reptiles have no members. In the whole exterior covering there is the greatest diversity, the interior conformation being nearly the same; they have all a heart, a liver, a stomach, intestines, and organs for generation; these ought to be considered as parts the most essential to the animal economy, since they are the most fixed, and least subjected to variation.