FINS OF ELASMOBRANCHS. 481 



and so grew into the developed cheiropterygium ; but there is not yet 

 enough evidence to determine what was the precise course of this trans- 

 formation. 



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

 pelvic girdle) through its need of a point (Tappui as a locomotive organ 

 on land. 



(12) The pelvic limb became also elongated; and when its function was quite 

 similar to that of the pectoral limb, its structure became also quite similar 

 (e. g. Ivhthyosmirus, Flesiosaunis, Chelydra, &c.) ; but for the ordinary qua- 

 drupedal mode of progression it became segmented and inflected in a way 

 generally parallel with, but (from its mode of use) in part inversely to, the 

 inflections of the pectoral limb. 



The amount of apparently spontaneous change needed for the processes enumerated 

 may appear excessive ; but I believe that in general we far too little appreciate the 

 excessive plasticity of the animal organism — a plasticity which results in, and is 

 evidenced by so many instances 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 the essence of which is " sen- 

 sitiveness " or " impressionability." Indeed an animal may be described as a more or 

 less complex arrangement for carrying about, nourishing, and perfecting a plexus 

 of sensations. 



In conclusion, then, the replies to our initial inquiries will be as follows : — 



A. The paired limbs and azygos fins are of similar origin and nature. 



B. Paired limbs are essentially peripheral structures which have become more or 



less closely connected with the skeletal axis. 



C. The limb-girdles are ingrowths from the bases of the limbs. 



D. The line of genesis of the cheiropterygium cannot yet be accurately determined. 

 (1) Vertebrate limbs are differentiations of continuous lateral folds ; they are 



therefore not limited to four, save for locomotive convenience ^. There might 

 be several successive paired limbs on each side, as there are sometimes several 

 successive dorsal fins, which are diflferentiations of a continuous dorsal fold. 

 Thus limbs and azygos fins are structures differing mainly by position, the 

 limbs being lateral, the azygos fins median. They may all be viewed as different 

 species (ichthyopterygia, cheiropterygia, dorsal fins, &c.) of one fundamental 



' The KcT. Dr. Haughton has shown good reason why the number should he thus limited iu most land 

 vertebrates. I do not at all understand why the lateral fold should not sometimes have resulted in the forma- 

 tion of more than four limbs in Fishes. 



3x2 



