THE CHAIN OF SPECIES. 577 



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 neurohaemal 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 ns see if we cannot trace some 

 homologues ? Without this historical derivation from annulose seg- 

 ments there will remain many things in cerebral anatomy entirely in- 

 explicable. To pursue this theme no further at present, the approxi- 

 mation 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 skel- 

 eton — 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 articu- 

 lata 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 diiFer essentially from that in the perfectly equilateral mol- 

 lusks, as in area, for instance, 'except in the nearer contiguity of the 

 ganglia. Finally, comparing area with lepas and cypris, it is mani- 

 fest 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 metamor- 

 phoses of the segments be correct, the homologue of the ganglia of 

 muscular motion, of the two principal valves of cypridae, 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 sys- 

 tems apparently peculiar. 



There is another branch of the internal structure of vertebrates in- 

 explicable upon any other hypothesis than this chain of specific descent. 

 I mean the haemal system — the system appropriated to the circulation 

 VOL. V. — 37 



