ept. 6, 1877] 



NATURE 



407 



the remains of continuous lateral fins. The professor then went 

 on to say : — 



The vertebrate animal is primarily composed of a cha'n of 

 similar segments, and there is no .! priori reison in morpholigy 

 why any one metamere should not b^ar limbs as wt^U as any 

 other. Nay, from the analogy of chas'oood worms we miylit 

 expect that — as in these each zonite iisually hears two pairs of 

 parapodia or stumpy foot-processes — so in similarly derived and 

 similarly segmented forms there might be at least traces of a 

 similar multiplication of appendages. 



In effect we really do find a somewhat parallel series in the 

 metaroeres of fishes, for, as Mr. Balfour has shown, the medio- 

 dorsal fin comes into existence precisely in the same manner as 

 the lateral fin ridge, and being a double structure, as we leirn, 

 both in its specialised form and even in the structure of the 

 cartilaginous precursor of the interspinous bones, it may reason- 

 ably be supposed to represent structures homologous with the 

 system of notopodia in a latei ally-compressed worm, fused toge- 

 ther, while the paired fins may be regarded as the neuropodin, 

 separated by the visceral caviiy, and which, in the degraded and 

 compressed metameres behind the visceral cavity, also coalesce, 

 forming ano'her primary ridge, that of the anal and caudal fin. 



In relation to the primary source of origin and method of 

 derivation of Inabs we have to account for two separate factors, 

 the Iraib-girdle and the lirab-rays ; with regard to the former I 

 can now only refer to the hypothesis of Gegenbaur and Dohrn, 

 that the limb-girdles represent modifications of the visceral 

 arches, and I pass this by with two comments : — I . That the 

 visceral arches are themselves to a certain extent specialised, and 

 consequently it would be better to state the hypothesis thus, that 

 the liinb-girdles and visceral arches are specialisations of corre- 

 sponding paraxial structures in different metameres. 2. In the 

 light ol the evident fundamental complexity of the limb-girdles 

 it seems a simpler explanation of phenomena to regard each 

 girdle as made up of the arches of several, probably three or 

 more, metameres fused, rather than as subdivisions of a single 

 arch. 



As to the primary nature of the limb-ray. Professors Huxley 

 and Gegenbaur have taught us in their recent reconstruction of the 

 theory of the Archipierygmm, that the primitive limb was con- 

 structed somewhat like the limb-ray of Ceratoius, having a 

 central jointed axis from which diverge fore and aft lateral pro- 

 cesses, or, to use the elegant nomenclature of Prof. Huxley, the 

 primitive vertebrate limb consisted of a column of mesomeres, 

 to eaLh of which a lateral pre- and post-axial paramere was 

 articulated. 



But even this form, though doubtless the stock from which 

 the limbs of all vertebrates above the Dipnoi have sprung, is 

 regarded, and with reason, by Gegenbaur as a derivative one, 

 formed by the coalescence of a still more archaic arrangement 

 of rays appended to the paraxial arches referred to above. It is 

 possible that the primary fusion may have taken such a form as 

 that which Gegenbaur represented in his original Archipterygium 

 with more than one cartilage appended to the girdle, a form of 

 which the arrangement in the dogfish and angel-shark may be 

 representative, and these, by a still farther concentration, attended 

 with an exaltation of the mesopterygium and a displacement of 

 the propterygium as in Hexanchus, or of the pro- and meta- 

 pterygium as in Cestracion, may thus reach the elongated form 

 of the limb in Dipnoi . It seems obvious that this fish Ceratodus, 

 though singularly genetalised, has arisen from a point in the 

 vertebrate stem above the starting-point of the elasmobranchs. 



Whether this has been the case or no, whether the elasmo- 

 branch has been derived from an earlier condition than the 

 dipnoan progenitor or no, the researches of Prof. Huxley have 

 made it plain that it is from the meso-, and not from the meta- 

 ptervgium that the single basal ray bone of the higher vertebrates 

 has arisen. 



A curious question will naturally occur to anyone considering 

 the genesis of limbs. What is the reason that in vertebrate 

 animals the number of limbs is limited, and apparently has 

 been always limited, to four ? and as we have seen that there is an 

 ontokigical possibility that each of these contains elements from 

 several metameres, there is no morphological reason, and there- 

 fore must be some mechanical cause for this limitation. Were 

 the primitive vertebiates terrestrial we could understand that the 

 tetrapod has a mechanical advantage over the tripod or any con- 

 dition with an inferior number of limbs, both statically from the 

 indeterminateness of the strain on each support in the four-legged 

 form, and in progression, from the easily understood conditions 

 of stabiUty of equilibrium in walking ; while the tetrapod excels 



the hexapod or millipede not only because, by a reduction in 

 number, the amount of nutrition required for the use of the limbs 

 is minimised, but it is absolutely demonstrable that the facility 

 of rotation is increased by the reduction of the limbs to the 

 lowest number consistent with other conditions of utility. In 

 connection with this point Prof. Haughton has made some 

 curious observations, the results of which I hope we shall have 

 laid before us in thi>. department during our present meeting. 



But the earliest vertebrates were aquatic, and yet even here 

 we find the four-fold division of these actival appendages. 

 These primitive forms differed from worms, in the greater 

 amount of fusion of their metameres, which at an early period 

 had ceased to give to these animals an externally jointed appear- 

 ance, so we may leain from Amphioxus, which has branched 

 from the veitehrate stem long before most of the secondary 

 characters, which are constant throughout the rest of the verte- 

 brates, had been foreshadowed. Being thus more consolidated 

 thm worms, and moving, as they would ni-cessarily do, m re as 

 a unit and less as a chain, the advantages of the moiHe of propul- 

 sion by a tail, over swimming by means of the continuous lateral 

 fin of united parapotia, would be increasingly manifest with 

 increasing somatic rigidity. Hence, naturally, the parapodia of 

 the hinder somites would coalesce to form a tail as ihey have 

 done in fishes, and the appendages placed far'her forward would 

 undergo retrogression unless some function could be found for 

 them which wou'd make their retention an advantage in the 

 economy. In the long worm-like firms like lam jreys, such a 

 retrogression has absolutely taken place, as in fishes of rhis form 

 the use of lateral fins is reduced to a minimum ; hence in the 

 elont;ated form of ordinary fishes, like eels, hand fishes, and 

 blennies, the lateral fins become radimental or varied. But 

 these organs are of obvious use in giving a capacity to 

 alter the plane of motion, a power which is necessary 

 for most fishes, as they only act in elevation and depres- 

 sion but in lateral rotation, as any one can verify for himself 

 by watching fishes in an aquarium. In order to accommo- 

 date these united lateral appendages mo.st conveniently to the 

 sinuous curves into which the body of a fish is thrown in swim- 

 ming, and to diminish the surface of resistance to the water, the 

 parapodia have divided themselves into two groups, leaving the 

 centre of the body, where the cephalic and caudal curves meet 

 during progression, free from lateral appendages. 



The address concluded by a brief notice of work in human 

 anatomy. It was remarked that much remained to be done in 

 details even in this well-wrought department. We had not 

 available correct and broadly based statements regarding the 

 average conditions of the variable parts of the human body. 

 Considering that something like six hundred bodies were annually 

 dissected in Great Britain and Ireland, such a basis ought to be 

 easily attainable. In such work Prof. Wenzel Griiber, of St. 

 Petersburg, the prince of descriptive anatomists, was laborious 

 and indefatigable. Yet still much remained to be done to make 

 human anatomy a really scientific study, a practical application 

 of morphological principles. 



One of the features of this department was Prof. Haughton's 

 discourse on the best possible number of limbs for an animal. 

 The criterion was the most economical expenditure of force, 

 according to mathematical principles. With regard to land 

 animals, it was shown that the three-limbed vertebrate is superior 

 to two and one-limbed forms, being perfectly stable when resting 

 on the limbs. The four-limbed vertebrate, however, was able 

 to preserve perfect stability on any three of its limbs, while 

 using the fourth for offence. There was for the most part no 

 advantage in having five or a greater number of limbs, the cost 

 of feeding the extra limb outweighing the advantage of possessing 

 it. Arboreal animals developed the tail as a filth limb because 

 of its special advantage in the medium in which they lived. 

 Man gave up the superior stability of having four limbs for 

 support in exchange for two adapted to higher uses, and in 

 correlation with his brain-power. In discussing the limbs of 

 aquatic .animals. Prof. Haughton showed how advantage in the 

 use of force and in directing motion was gained by concentration 

 of motor power and limitation of the number of limbs. The 

 three-limbed and one-limbed swimmers had advantages over all 

 others, and odd-limbed forms over even-limbed. In many 

 respects the three-limbed swimmers were superior to the one- 

 limbed, but the latter had the valuable quality of being able to 

 apply a maximum force in one direction, and so escape from 

 their enemies. Prof. H.iughton indicated fishes [as really pos- 



