A similar gradation is observed in the development of the various parts composing the hand and arm in animals with perfect skeletons, in which the bones act as levers. There is a very interesting book[C] written on this subject, which you would do well to read; and you will see in it that all sorts of birds, quadrupeds, and even fishes, have, in their fore feet, wings, and fore fins, elements resembling those of the human hand and arm. Often, in comparing only two animals, you might fail to trace the slightest resemblance; but when one or two or more of other kinds are placed between them, you find a sort of ladder which, in an evident manner, unites the lowest with the highest tribes. [Footnote C: Sir Charles Bell's Treatise on the Hand.] At distant periods of the earth's history, you may see the same sort of organs, and the same dependance of the creatures one on another. The fierce Icthyosaurus, and the sly, long-necked Plesiosaurus, had eyes, ears, tongues, and other parts, the same as our contemporaries; and they ate and digested their food, and moved from place to place, and preyed on each other in no other manner. If our acquaintance with nature were much greater than it is, we should doubtless be able to bring proofs that there is no sort of stones, of vegetables, or of animals, nor any process or movement of the elements, in which we have not an interest. There is no fact that is not in some way or other connected with the whole, so as to influence its well-being. VII. Not only are the creatures which inhabit the earth united together by bonds of similarity of structure and appetite, of common wants and enjoyments, and of mutual support; but we are also united with the boundless system of worlds which the night unveils to our view. The principle of gravitation, and the beneficent rays of the sun operating on the planets and their moons, throw over us a plain and obvious tie of brotherhood with the stars that we may see night after night making their way amongst the constellations, as they move in their orbits, distinguished by their steady light from the twinkling multitude of fixed stars. The law that unites us to them is the same as causes a drop of rain to descend, or a weary fly to settle on the earth. It is not improbable, that the aerolites (if the theory which I hinted at in a former chapter be true) may be fragments of original matter, which have never been appropriated by any globe, and now sometimes pitch on one planet, and sometimes on another. If this be the case, we should be warranted in concluding that the matter of our solar system is everywhere the same, chemically considered, and is, therefore, governed by the same chemical laws; for the aerolites contain no substance which is not to be found far below the surface of the earth.