3IO 



NATURE 



[July 1 8, 1878 



But the action of an internal polar force is evidenced 

 by other phenomena of pathology and teratology, which' 

 come to the aid of comparative anatomy in demonstrating 

 its existence. Thus w e very often see that parts which 

 are serially or bilaterally symmetrical, are abnormally 

 affected in a similar manner,' as recorded by Sir James 

 Paget, Dr. William Budd, Prof. Burt Wilder, and others, 

 and the same is the case with congenital deformities.''' 



Perhaps, however, the most curious and instructive 

 are those presented by some of our domestic birds. In birds 

 we have found the serial symmetry of the limbs reduced 

 almost to its minimum — the leg and foot being so widely 

 different from the wing. In trumpeter pigeons and some 

 bantams, however, the feet, which usually are naked, 

 become abnormally furnished with feathers (technically 

 called "boots"), which may be even longer than the 

 wing feathers, and are developed from that side of the foot 

 which corresponds with the feather-bearing side of the 

 hand. Moreover, in ordinary pigeons, though the digits 

 of the hand are completely united together, while the toes 

 of the foot are free, but in " booted ' ' pigeons the outer 

 toes become more or less united together by skin like the 

 fingers. 



With facts such as these it seems to m€ unreasonable 

 to deny the existence in each animal (which as a whole is 

 a visible unity) of an innate polar force tending to carry 

 out development in definite directions, but liable to have 

 its action and effects modified by the environment. 



We may now turn to those other initial questions. 

 Whence have limbs such as ours arisen? What is a 

 limb? 



As to the first question : The digit-bearing limbs of 

 man, beasts, birds, reptiles, and batrachians are usually 

 supposed to have been derived from structures having 

 more or less resemblance to the paired fins of fishes, but 

 the path which the genetic process has followed is dif- 

 ferently represented by different evolutionists. 



As to the second question, i.e., as to the essential 

 nature of vertebrates' limbs, of whatsoever kind, different 

 views have also been maintained. By some anatomists 

 they have been regarded as parts which have in one way 

 or another been derived from the axial skeleton, by 

 others they have been represented as skeletal structures 

 appended to, but not derived from the axial skeleton. 



In 1843 Oken taught the extremely fanciful doctrine 

 that arms and legs are so many liberated ribs, and Carus 

 followed him to a certain extent, teaching that they are 

 essentially elements radiating from the exterior of a rib- 

 like arch. 



In 1848 Owen propounded the view that they are 

 diverging appendages attached to ribs like the uncinate 

 processes of birds, and he compared them to the 

 branchiostegal rays of fishes. He also taught that the 

 shoulder and pelvic girdles are modified rib arches. 



In 1852 Maclise represented the limbs as modified 

 ribs, the parts beyond the elbow and the knee, however, 

 corresponding with the interspinus bones and fin rays of 

 fishes' azygos fins. 



In 1857 Goodsir described them as radiating actino- 

 pophyses.^ 



In 1 87 1 Humphrey represented the limbs as modified 

 portions of a primitively continuous inferior azygos fins. 



In 1872 Gegenbaur threw out the suggestions which 

 he has since (1874 and 1876) more definitely adopted, 

 that the shoulder girdle is a modified arch of similar 

 nature to the branchial arches, the limbs having been 

 formed from rays diverging outwards from such an arch. 

 He also considers the skeleton of the azygos fins as the 

 separated ends of the neural and hoemal spines of the 

 vertebral column.* 



Owen and Gegenbaur consider the paired and azygos 



See " Genesis of Species," second edition, p. 205. 

 ^ Loc. cit., p. 202. 



3 Edinburgh New Phil. Journal, vol. v. (new series), 1857, p. 178. 

 * ' Grundr.ss d. Vergl. Anat.," 1874, p. 488. 



limbs to be two fundamentally different structures, and 

 Huxley ' calls their sustaining bones or cartilages " ele- 

 ments of the exoskeleton." 



The fundamental distinctness between the paired limbs 

 and the axial system appears to have been held by 

 Cuvier and by Huxley. I advocated the same view in 

 1870,- and I have since expressed^ my conviction "that 

 the appendicular skeleton is no mere portion of the axial 

 skeleton, but a distinct system of parts appended to, and 

 more or less closely and variously connected with, the axial 

 system.' ' To this conviction I now adhere more firmly than 

 ever. I have also been long convinced that the shoulder- 

 girdle could not be (as Gegenbaur thinks) a branchial arch, 

 or be formed of coalesced branchial arches, as also that the 

 branchial arches could not be (as some have supposed) 

 serially homologous with rib-arches. For the branchial 

 arches are within the aortic vessels, which vessels I took* to 



Fig. 16.— Diagram of the condition of the skeleton in the branchial region 

 of some sharks (transverse vertical section). — n, neural canal ; b, ali- 

 mentary canal, surrounded by A, the branchial arches (splanchnapo- 

 physes) ; v, the aortic vessels, extending up outside the branchial arches 

 and inside the paraxial system (/), here represented by certain external 

 branchial cartilages. 



" indicate the line of the pleuro-peritoneal division of the 

 ventral laminae" of the embryo. This conviction has 

 been remarkably justified by Mr. Balfour's recent dis- 

 covery of the continuation (in embryo elasmobranchs) of 

 the pleuro-peritoneal cavity into the head, and externally 

 to their aortic vessels.^ 



Prof. Parker has recently suggested^ that the "ex- 

 trabranchials " of the dogfish may be homologous with 

 the scapulo-coracoid, but I am persuaded this is not the 

 case. 



In 1876 Mr. Balfour described' the development of 

 the Hmbs of elasmobranchs as "special developments ot 

 a continuous ridge on each side, precisely like the ridges 

 of epiblast which form the rudiments of the unpaired 

 fins." Since then the paired fins arise in the same way 

 as the azygos fins ; they all probably belong to the same 

 category of peripheral, non-axial structures. Moreover, 

 since this is the mode of origin in the individual, there 

 of course arises an d, priori probability that it was the 

 mode of origin in the race, and that the primeval verte- 

 brate limbs were a pair of continuous lateral folds, serving 

 to balance the body in swimming. 



Now I conceive it will hardly be disputed that when 

 supporting hard structures were first developed in the 

 azygos fins they had the form they so generally still 

 present, of a longitudinal series of numerous similar, 

 separate, rod-like structures. But the skeleton of the 

 paired, especially of the pectoral fins, is very different 

 from this. The interesting questions, therefore, arise : (i) 

 Whether any azygos fins present characters approxi- 

 mating them to the normally developed paired fins ? and 

 (2) Whether any paired fins present characters approxi- 

 mating them to the normally developed azygos fins ? 



A detailed aceountof some recent dissections of fish-fins 

 made with a view to answer these questions — dissections 

 effected through the kindness and liberality of Dr. Giinther, 

 at the British Museum— have been lately ^ communicated 

 to the Zoological Society. The result of those dissections 



' " Anat. of Vertebrates," p. 43. 



2 Linn. Trans., vol. xxvii. p. 388. 



3 " Lessons in Elementary Anatomy," 1873, p. 230. 



♦ Linn. Trans., I.e. _,, .1 , 



5 Cambridge Journal, vol. xi. part 3. Apr.I, 1877. P- 474- The authors 

 words are :— " It occupies a position on the outer side of the aortic trunic or 

 its arch." 



6 " Moiphology of the Skull," p. 343- . „ , ^ ^0,0 



7 Cambridge Journal, vol. xl. part i,p. IS^- ' *°'- ^- "?<>• 



