In the fact, also, that we find in fossil specimens organs of respiration, such as lungs, gills, and trachea, we learn that the process of a circulation of blood, and its purification by means of the oxygen of the atmosphere, have never varied. Animal heat, too, dependent as it is essentially upon this oxygenating process, was always derived from the same source as at present.
The perfectly preserved minute vessels of vegetables enable us, by means of the microscope, to identify them with the plants now alive; and they prove, too, incontestably, that the nourishment of vegetables has always been of the same kind, and has been converted into the various proximate principles of plants by the same processes.
Again. We have evidence that these ancient animals possessed the same senses as their congeneric races now on the globe. We have one good example in which that most delicate organ, the eye, is most perfectly preserved. It is well known that the visual organ of insects and of crustaceans is composed of a multitude--often several hundreds or thousands--of eyes, united into one, so as to serve the purpose of a multiplying glass; each eye producing a separate image of the object observed. Such an eye had the trilobite. Each contained at least four hundred nearly spherical lenses on the surface of the cornea, united into one organ; revealing to us the interesting fact, that the relations of light to animal organization were the same in that remote era as they now are.
But I need not multiply proof of the functional identity of organic nature in all ages. It may, however, be inquired, how this identity, as well as that of anatomical structure, is reconciled with the great anomalies, both in size and form, which have confessedly prevailed among ancient animals. Compare the plants and animals which now occupy the northern parts of the globe with those which flourished there in the remote periods of geological history, and can we believe them to be portions of one great system of organic nature?
Compare, for instance, the thirty or forty species of ferns now growing to the height of a few inches, or one or two feet, in Europe and this country, with the more than two hundred species already dug out of the coal mines, many of which were forty to forty-five feet in height; or the diminutive ground pines, and equiseta, now scarcely noticed in our forests, with the gigantic lepidodendron, sigillaria, calamites, and equiseta, of the carboniferous period; and who will not be struck with the great difference between them?