Chapter LII: Appendix: D

ON THE ORIGIN OF THE VERTEBRATE TYPE.

[_When studying the development of the vertebrate skeleton,
there occurred to me the following idea respecting the possible
origin of the notochord. I was eventually led to omit the few
pages of Appendix in which I had expressed this idea, because
it was unsupported by developmental evidence. The developmental
evidence recently discovered, however, has led Professor Haeckel
and others to analogous views respecting the affiliation of the_
Vertebrata _on the_ Molluscoida. _Having fortunately preserved a
proof of the suppressed pages, I am able now to add them. With
the omission of a superfluous paragraph, they are reprinted
verbatim from this proof, which dates back to the autumn of
1865, at which time the chapter on “The Shapes of Vertebrate
Skeletons” was written._--December, 1869.]

The general argument contained in Chap. XVI. of Part IV., I have thought it undesirable to implicate with any conception more speculative than those essential to it; and to avoid so implicating it, I transfer to this place an hypothesis respecting the derivation of the rudimentary vertebrate structure, which appears to me worth considering.

Among those molluscoid animals with which the lowest vertebrate animal has sundry traits in common, it very generally happens that while the adult is stationary the larva is locomotive. The locomotion of the larva is effected by the undulations of a tail. In shape and movement one of these young Ascidians is not altogether unlike a Tadpole. And as the tail of the Tadpole disappears when its function comes to be fulfilled by limbs; so the Ascidian larva’s tail disappears when fixation of the larva renders it useless. This disappearance of the tail, however, is not without exception. The _Appendicularia_ is an Ascidian which retains its tail throughout life; and by its aid continues throughout life to swim about. Now this tail of the _Appendicularia_ has a very suggestive structure. It is long, tapering to a point, and flattened. From end to end there runs a mid-rib, which appears to be an imbedded gelatinous rod, not unlike a notochord. Extending along the two sides of this mid-rib, are bundles of muscular fibres; and its top bears a gangliated nervous thread, giving off, at intervals, branches to the muscular fibres. In the _Appendicularia_ this tail, which is inserted at the lower part of the back, is bent forwards, so as not to be adapted for propelling the body of the animal head foremost; but the homologous tails of the larval Ascidians are directed backwards, so as to produce forward movement. If we suppose a type like the _Appendicularia_ in the structure and insertion of its permanent tail, but resembling the larval forms in the direction of its tail, it is, I think, not difficult to see that functional adaptation joined with natural selection, might readily produce a type approximating to that whose origin we are considering. It is a fair assumption that an habitually-locomotive creature would profit by increased power of locomotion. This granted, it follows that such further development of the tail-structures as might arise from enhanced function, and such better distribution of them as spontaneous variation might from time to time initiate, would be perpetuated. What must be the accompanying changes? The more vigorous action of such an appendage implies a firmer insertion into the body; and this would be effected by the prolongation forwards of the central axis of the tail into the creature’s back. As fast as there progressed this fusion of the increasingly-powerful tail with the body, the body would begin to partake of its oscillations; and at the same time that the resistant axis of the tail advanced along the dorsal region, its accompanying muscular fibres would spread over the sides of the body: gradually taking such modified directions and insertions as their new conditions rendered most advantageous. Without further explanation, those who examine drawings of the structures described, will, I think, see that in such a way a tail homologous with that of the _Appendicularia_, would be likely, in the course of that development required for its greater efficiency, gradually to encroach on the body, until its mid-rib became the dorsal axis, its gangliated nerve-thread the spinal chord, and its muscular fibres the myocommata. Such a development of an appendage into a dominant part of the organism, though at first sight a startling supposition, is not without plenty of parallels: instance the way in which the cerebral ganglia, originally mere adjuncts of the spinal chord, eventually become the great centres of the nervous system to which the spinal chord is quite subordinate; or instance the way in which the limbs, small and inconspicuous in fishes, become, in Man, masses which, taken together, outweigh the trunk. It may be added that these familiar cases have a further appropriateness; for they exhibit higher degrees of that same increasing dominance of the organs of external relation, which the hypothesis itself implies.

Of course, if the rudimentary vertebrate apparatus thus grew into, and spread over, a molluscoid visceral system, the formation of the notochord under the action of alternating transverse strains, did not take place as suggested in § 255; but it does not therefore follow that its differentiation from surrounding tissues was not mechanically initiated in the way described. For what was said in that section respecting the effects of lateral bendings of the body, equally applies to lateral bendings of the tail; and as fast as the developing tail encroached on the body, the body would become implicated in the transverse strains, and the differentiation would advance forwards under the influences originally alleged. Obviously, too, though the lateral muscular masses would in this case have a different history; yet the segmentation of them would be eventually determined by the assigned causes. For as fast as the strata of contractile fibres, developing somewhat in advance of the dorsal axis, spread along the sides, they would come under the influence of the alternate flexions; and while, by survival of the fittest, their parts became adjusted in direction, their segmentation would, as before, accompany their increasing massiveness. The actions and reactions due to lateral undulations would still, therefore, be the causes of differentiation, with which natural selection would co-operate.