600 APPENDIX D. 



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 in- 

 creased 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 pro- 

 longation 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 de- 

 velopment 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. 



