446 EMBRYOLOGY OK TIIK Lo\VKR VERTEBRATES CH. 



Sharks and Dog-fishes do not. so Car as is known, develop this 

 ridge. 



(2) The fact that the myotomes from which the linili rudiment, 

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

 voluntary muscles aiv themselves nietameric and that the skeletal 

 elements necessarily correspond in .position with the muscles seems 

 in render it unnecessary to seek any further evolutionary explana- 

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

 the limit to exhibit a markedly nietameric 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 tailward 

 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 

 linib. Hut 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 tin 

 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 tin- 

 evolutionary origin of the limbs. 



As regards anatomical evidence stress is laid on the exceedingly 

 ' -lose 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 arrangements may quite probably be merely a oaae 

 of that secondary convergence of which so many striking examples 

 are known in the annual kingdom, 



An ancient fossil fish. Cladoselaclie, is brought in to corroborate 

 the view, it- paired lins having each a broad longitudinally-running 

 base ol' attachment and being apparently supported by separate 

 without any continuous ba>al skeleton. lint it- is pointed out (1) 

 that what BlgM I here a i of basal skelel on may he readily interpreted 

 presenting the axis of a tin of tin- ('eralndus type laid hack 



against tin- side of the i,,dy I L69 E, p. 353), and (2) that 



riielinv of the tail I- of a Vety highly developed Mild powerful 



Ivpe and that it is most unlikely that a powerful s\\ imnier, such as 

 ih.- hi'jlilv \ol\vd tail demonstrates f 'l,i,/,isel,n-h,- to have been, should 



