THE CHAIN OF SPECIES. 



S7S 



and also of two segments of any annulose animal — a crustacean or 

 a centipede, for instance ; and when we have seen m the progress from 

 unicellular and from monosegmental structures, to the multi-segmen- 

 tal; and from these again to the bisegmental, by lopping olT 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 becom- 

 ing the normal conditions of some creatures — all mystery vanishes. 



First, let us notice that the so-called vertebrate segments or sec- 

 tions — the vertebrae 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 vertebrae 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 de- 

 pends 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 in- 

 stance, 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 articulae. 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 aeration 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 verte- 

 brate skeleton by observing two distinct systems of organization — 

 the dermal and the neurohoBmal, heretofore supposed to be peculiar to 

 vertebrates. Now, to which system, the dermal or the neurohsemal, 

 do the extremities belong ? No amount of ingenuity can satisfy the 

 thoughtful student that the limbs are evolutions of the appendages of 

 the neurohaemal axis. They are dermal. Even in mammalia they 



