But, it may be objected, this is only of external matters, the limbs, the appendages; and these may be accidental correspondences.
Very well! Select any other set of organs. Are there any more internal and peculiar to vertebrates than the brain and nerves? Let us take the neurohæmal system itself, that which confessedly has no perfect counterpart in the articulate class. It is not suggested that any creature lower than cephalopod mollusks presents any thing exactly corresponding to the cerebral hemispheres, and the optic and auditory lobes of the vertebrate brain. But let us see if we cannot trace some homologues? Without this historical derivation from annulose segments there will remain many things in cerebral anatomy entirely inexplicable. To pursue this theme no further at present, the approximation of the vertebrate and the molluscan systems of nerves cannot be doubted. In higher cephalopods, as sepia, the principal ganglia are brought into near proximity—at least resembling a brain. They are even covered by a bony framework, rudimentary of a neural skeleton—in all of which the great cephalopod is decidedly more cerebrate than some fishes, to say nothing of the doubtful amphioxus. But if all else were wanting, the auditory lobes and the optical apparatus would establish this correspondence with the highest orders. The cuttle-fish has an eye with a retina, lens, iris, and cornea; and the optical ganglia are as truly lobes of the brain as they are in mammalia. Nothing like this is seen in acephalous mollusks, nor in articulata. Where articulata have a machinery for vision, it is not organized upon this plan. But we do see centres of nerve-force, or ganglia, appertaining to every segment; and we do see also that the nervous system of sepia is only an advance upon that of the inferior mollusca. Even in the oyster the ganglia are brought nearly to a common centre; and this arrangement does not differ essentially from that in the perfectly equilateral mollusks, as in area, for instance, except in the nearer contiguity of the ganglia. Finally, comparing area with lepas and cypris, it is manifest that we have essentially the same plan as in the nervous system of any two segments of articulata. For you will find in each segment two ganglia, one on each side of the median line; and these being brought together, as they generally are in annulosa, appear as one. Now, this one twofold ganglion is, if our explanation of the metamorphoses of the segments be correct, the homologue of the ganglia of muscular motion, of the two principal valves of cypridæ, of cirrhipeds, and of bivalve mollusca; and finds its final evolution in the quadruple structure of a mammalian brain. In fact, the homology is complete, with the additions, or rather modifications, already explained, which endow vertebrates with olfactory, auditory, and optic lobes and systems apparently peculiar.
There is another branch of the internal structure of vertebrates inexplicable upon any other hypothesis than this chain of specific descent. I mean the hæmal system—the system appropriated to the circulation
