and also of two segments of any annulose animal—a crustacean or a centipede, for instance; and when we have seen in the progress from unicellular and from monosegmental structures, to the multi-segmental; and from these again to the bisegmental, by lopping off surplus machinery, and by modifying and enlarging what is left in order to increase and intensify the forces evolved; when we actually see these changes, with the attendant change of axis, taking place and becoming the normal conditions of some creatures—all mystery vanishes.
First, let us notice that the so-called vertebrate segments or sections—the vertebræ of the spinal column, are not segments in the sense used here—are not segmenta, the biological bracelets, in any true sense; although their development takes place after the change of axis by a growth analogous to the original evolution of annulose segments. The vertebræ are not homologues of the segments of the great segmentarian series.
Having thus indicated what the segments are not, let us call to mind again what they are. This will be the best way to establish the chain we are seeking to comprehend, as it will also be the readiest plan of exhibiting the homologues of these classes, upon which depends all our knowledge of transcendental biology. It is best done, in the few moments left us, by tracing a few of these homologues. For this purpose let us select prominent and obvious organs, for instance, the heart, the brain, and the extremities or appendages.
The first to be noted, being most obvious to popular inquiry, are the extremities or limbs—the true articulæ. Fins, wings, legs, and arms—can these truly be homologues of the arthritic appendages of annulosa? They are indeed. Not half so plain, when first announced, was that first wonderful revelation of comparative anatomy, which displayed the fin of a fish, the paddle of a whale, the wing of a bird, the leg of a horse, and the arm of a man, to be composed of the same organic elements, as is this which now proclaims that these so varied and beauteous limbs of vertebrate animals made the first essays of their evolution as the lateral appendages of aëration or locomotion of the segments of the lowest orders of annulosa.
The proofs of this are now obvious, but to appreciate them we have need to travel back again through all the grades of life, and weigh the homologues of every organic element.
Yet some of the evidences are plain, and almost superficial, in the vertebrate class itself. Every anatomist, in order to comprehend the physiology and anatomy of this class, begins his analysis of the vertebrate skeleton by observing two distinct systems of organization—the dermal and the neurohæmal, heretofore supposed to be peculiar to vertebrates. Now, to which system, the dermal or the neurohæmal, do the extremities belong? No amount of ingenuity can satisfy the thoughtful student that the limbs are evolutions of the appendages of the neurohæmal axis. They are dermal. Even in mammalia they
