TRANSACTIONS OF THE SECTIONS. 93 



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 archipterygium, 

 that the primitive limb was constructed somewhat like the limb-ray of Cerutodus, 

 having a central jointed axis from which diverge, fore and aft, lateral processes, 

 or, to use the elegant nomenclature of Professor Huxley, "the primitive vertebrate 

 limb consisted of a column of mesomeres, to each 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 ver- 

 tebrates 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 Dog-fish and Angel-shark may be 

 representative; and these, by a still further concentration, attended with an exal- 

 tation of the mesopterygium and a displacement of the propterygium as in Ilcv- 

 anchus, or of the pro- and metapterygium, as in Cestracion, may thus reach the 

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

 though singularly generalized, has arisen from a point in the vertebrate stem above 

 the starting-point of the Elasmobranchs. 



Whether this has been tho case or no, whether the Elasmobranch has been de- 

 rived from an earlier condition than the Dipnoan progenitor or no, the researches 

 of Professor Huxley have made it plain that it is from the meso- and not from the 

 rnetapterymmi that the single basal ray-bone of the higher vertebrates has arisen. 



A curious question will naturally occur to any one 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 ontological possibility that each of these contains elements from 

 several metanieres, there is no morphological reason, and therefore must be some 

 mechanical cause, for this limitation. Were the primitive vertebrates terrestrial, 

 we could understand that the tetrapod has a mechanical advantage over the tripod 

 or any condition with an inferior number of limbs, both statically, from the inde- 

 terminateness of the strain on each support in the four-legged form, and in pro- 

 gression, from the easily understood conditions of stability of equilibrium in walk- 

 in^ • while the tetrapod excels the hexapod or millepede, not only because, by a 

 reduction in number, the amount of nutrition required for the use of the limbs is 

 minimized, 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 connexion with this point, Professor Haughton has made some 

 curious observations, the results of wliich I hope we shall have laid before us in 

 this department during our present meeting. 



But the earliest vertebrates were aquatic ; and yet even here we find the four- 

 fold division of these actinal appendages. These primitive forms differed from 

 worms in the greater amount'of fusion of their metanieres, which at an early period 

 had ceased to°give to these animals an externally jointed appearance, as we may 

 leam from Amphioxus, which has branched from the vertebrate stem long before 

 most of the secondary characters, which are constant throughout the rest of the 

 vertebrates, had been foreshadowed. Being thus more consolidated than worms, and 

 movin"-, as they would necessarily do, more as a unit and less as a chain, the ad- 

 vantages of the mode of propulsion by a tail over swimming by means of the con- 

 tinuous lateral fin of united parapodia would be increasingly manifest with in- 

 creasing somatic rigidity. Hence, naturally, the parapodia of the hinder somites 

 would coalesce to form a tail, as they have done in fishes, and theappendages 

 placed further forward would undergo retrogression, unless some f miction could bo 

 "found for them which would make their retention an advantage in the economy. 

 In the Ion"- worm-like forms like Lampreys, such a retrogression has absolutely taken 

 place as in fishes of this form the use of lateral fins is reduced to a minimum ; 

 hence' in the elongated form of ordinary fishes, like Eels, Band-fishes, and Blennies, 

 the lateral fins become rudiniental or vanish. But these organs are of obvious use 

 in giving a capacity to alter the plane of motion, a power which is necessary for 



