1878.] PROF. MIVART ON THE FINS OF ELASMOBRANCHS. 119 



lateral fold so as to form two prominences on each side, i. e. the 

 primitive paired limbs. 



(4) Each anterior limb increased in size more rapidly than the 

 posterior limb. 



(5) The bases of the cartilaginous supports coalesced as was 

 needed according to the respective practical needs of the different 

 separate portions of the longitudinal folds, i. e. the respective needs 

 of the several fins. 



(b') Occasionally the dorsal radials coalesced (as in Notidanus &c.) 

 and sought centripetally (as in Pristis &c.) adhesion to the skeletal 

 axis. 



(7) The rays of the hinder paired limbs did so more constantly, 

 and ultimately prolonged themselves inwards by mesiad growths from 

 their coalesced bases, till the piscine pelvic structure arose as we see 

 it in Squat ina l 



(8) The pectoral rays with increasing development also coalesced 

 proximally, and, thence prolonging themselves inwards to seek a 

 point d'appui, shot dorsad and ventrad to obtain a firm support and 

 at the same time to avoid the visceral cavity. Thus they came to 

 abut dorsally against the axial skeleton and to meet ventrally together 

 in the middle line below. 



(9) The lateral fins, as they were applied to support the body on 

 the ground, became elongated, segmented and narrowed, so that 

 probably the line of the propterygium, or possibly that of the meso- 

 pterygium, became the axis of the digit-bearing limb. 



(10) The distal end of the incipient cheiropterygium either pre- 

 served and enlarged preexisting cartilages or developed fresh ones to 

 serve fresh needs, and so grew into the developed cheiropterygium ; 

 but there is not as yet enough evidence to determine what was the 

 precise course of this transformation. 



(11) The pelvic limb acquired a solid connexion with the axial 

 skeleton, a pelvic girdle, through its need of a point oVuppui as a 

 locomotive organ on land. 



(12) The pelvic limb became also elongated; and in those cases 

 where its function was quite similar to that of the pectoral limb its 

 structure became also quite similar {e.g. Icthyosaurus, Plesiosaurus, 

 Chelydra, &c); but for the quadrupedal mode of progression it 

 became segmented and inflected in a way generally parallel with, but 

 (from its mode of use) in most cases in part inversely to, the inflec- 

 tions of the pectoral limbs. 



The amount of apparently spontaneous change needed to effect 

 these transformations may appear excessive ; but I believe that the 

 excessive plasticity of the animal organism is generally too little ap- 

 preciated — a plasticity which results in and is evidenced by the many 

 instances we now know of the independent origin of similar structures. 

 The plasticity of animals might be expected to be great ; for plasticity 

 is bodily reaction in response to external stimuli. The response which 

 is most rapid and complete is sensation ; and an animal is a creature 



1 This view has been put forward by Mr. J. K. Thachor of New Haven, 

 Connecticut. See Connecticut Trans, vol. iii. 



