"One who could view," he writes, "Nature in her entirety, from first to last, and then reflect upon the manner in which these two substances--the living and the inanimate--act and react upon one another, would see that every living being is a mould which casts into its own shape those substances upon which it feeds; that it is this assimilation which constitutes the growth of the body, whose development is not simply an augmentation of volume, but an extension in all its dimensions, a penetration of new matter into all parts of its mass: he would see that these parts augment proportionately with the whole, and the whole proportionately with these parts, while general configuration remains the same until the full development is accomplished.... He would see that man, the quadruped, the cetacean, the bird, reptile, insect, tree, plant, herb, all are nourished, grow, and reproduce themselves on this same system, and that though their manner of feeding and of reproducing themselves may appear so different, this is only because the general and common cause upon which these operations depend can only operate in the individual agreeably with the form of each species. Travelling onward (for it has taken the human mind ages to arrive at these great truths, from which all others are derived), he would compare living forms, give them names to distinguish them, and other names to connect them with each other. Taking his own body as the model with which all living forms should be compared, and having measured them, explained them thoroughly, and compared them in all their parts, he would see that there is but small difference between the forms of living beings; that by dissecting the ape he could arrive at the anatomy of man, and that taking some other animal we find always the same ultimate plan of organization, the same senses, the same viscera, the same bones, the same flesh, the same movements of the fluids, the same play and action of the solids; he would find all of them with a heart, veins, arteries, in all the same organs of circulation, respiration, digestion, nutrition, secretion; in all of them a solid frame, composed of pieces put together in nearly the same manner; and he would find this system always the same, from man to the ape, from the ape to the quadrupeds, from the quadrupeds to the cetacea, birds, fishes, reptiles; this system or plan then, I say, if firmly laid hold of and comprehended by the human mind, is a true copy of nature; it is the simplest and most general point of view from which we can consider her, and if we extend our view, and go on from what lives to what vegetates, we may see this plan--which originally did but vary almost imperceptibly--change its scope and descend gradually from reptiles to insects, from insects to worms, from worms to zoophytes, from zoophytes to plants, and yet keeping ever the same fundamental unity in spite of differences of detail, insomuch that nutrition, development, and reproduction remain