446 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



Sharks and Dog-fishes — do not, so -far as is known, develop this 

 ridge. 



(2) The fact that the myotonies — from which the limh rudiment, 

 like other portions of the body, has to derive its equipment of 

 voluntary muscles — are themselves metameric and that the skeletal 

 elements necessarily correspond in position with the muscles seems 

 to render it unnecessary to seek any further evolutionary explana- 

 tion of the tendency on the part of the musculature and skeleton of 

 the limb to exhibit a markedly metameric appearance during early 

 stages in its development. 



The occurrence of abortive muscle-buds in front of the definitive 

 limb is taken — quite reasonably — as evidence pointing to a tail ward 

 shifting of the anterior -margin of the limb having taken place, and 

 similarly the presence of abortive buds behind the definitive limb is 

 taken as evidence of a headward shifting of the hinder margin of the 

 limb. Ihit this shifting of the anterior and posterior margins of the 

 limb may have in evolution taken place either synchronously {i.e. 

 together with a narrowing of the base of attachment of the fin) or at 

 different periods as the limb shifted backwards and forwards as a 

 whole in accordance with variations in adaptational requirements. 

 The present writer sees no convincing reason for rejecting either of 

 these possibilities — and if either be possible then the evidence loses 

 its value as support of one view rather than the other. 



(3) The narrowing of the limb proximally and its expansion 

 distally is a process which would naturally take place as' the fin 

 became more efficient as a propelling organ — just as in the evolution 

 of a racing oar or paddle . with its broad blade and slender shaft — 

 and, accordingly, too great weight should not be attached to the 

 occurrence of such a process during ontogeny in arguing as to the 

 evolutionary origin of the limbs. 



As regards anatomical evidence stress is laid on the exceedingly 

 close structural resemblance in skeleton and musculature between 

 the paired and the unpaired fins. On the other hand it is suggested 

 that, seeing that lateral and paired fins are organs similar in function 

 and built up out of similar muscular elements, a close similarity in 

 their anatomical arrangement's may quite probably be merely a case 

 of that secondary convergence of which so many striking examples 

 are known in the animal kingdom. 



An ancient fossil fish, Ctadoselache, is brought in to corroborate 

 the view, its paired fins having each a broad longitudinally-running 

 base of attachment and being apparently supported by separate rays 

 without any continuous basal skeleton. But it is pointed out (1) 

 that what signs there are of basal skeleton may be readily interpreted 

 as representing the axis of a fin of the Ceratodus type laid back 

 against the side of the body (see Eig. 169, E, p. 353), and (2) that 

 the structure of the tail is of a very highly developed and powerful 

 type and that it is most unlikely that a powerful swimmer, such as 

 the highly evolved tail demonstrates Cladoselache to have been, should 



