Their near equality of rank and use, as measured by the significance of form, over-rides all mechanical difference in their mode of working. The heart is, in office, a forcing pump or engine of the circulation. The lungs have no motion of their own, and the porosity or cellular formation of the sponge seems to be the only quality of texture that they require for their duty, which is classed as a process of vital chemistry. The brain differs, again, into a distinct category of function, which accepts no classification, but bears some resemblance to electrical action. Yet, differing thus by all the unlikeness that there is between mechanical, chemical and electro-vital modes of action, they evidently derive their very considerable resemblance of figure from their nearly equal elevation and dignity of service in the frame. This near neighborhood of use and rank allows, however, room enough for their individual differences and its marks. The heart is lowest of the three in rank, and nearest the regularly conical form. The lungs, as their shape is indicated by the cavity which they occupy, are more delicately tapered at their apex, and more oblique and variously incurvated at their base. And the brain, whether viewed in four compartments, or two, or entire, (it admits naturally of such division,) answers still nearer to the highest style and form of the life pattern; and with the due degree of resemblance, or allusion to it, in its several parts, according to their probable value; for the hemispheres are shaped much more conformably to the ideal than the cerebellum or the cerebral apparatus at the base of the brain, where the office begins to change from that of generating the nervous power to the lower service of merely conducting it out to the dependencies. IV. Hitherto we have looked for proof and illustration only to well marked and clearly defined examples of the orders and kinds of things examined. But the borders of kingdoms and classes, the individuals which make the transitions, and the elements and qualities common to several provinces which link kind to kind and rank to rank, confess the same law, and even more nicely illustrate where, to superficial view, they seem to contradict it. Every species of beings in the creation is a reproduction, with modifications and additions, but a real reproduction, in effect, of all that is below it in the scale; so that the simplest and the lowest continues and reappears in all, through all variety of advancement, up to the most complex and the highest; in some sense, as decimals include the constituent units, and hundreds include the tens, and other multiples of these embrace them again, until the perfect number is reached, if there be any such bound to either numerals or natures.