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 dappui 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. Ichthyosaurus, Plesiosaurus, 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. ‘lhe 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 differentiations 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 Key. Dr. Haughton has shown good reason why the number should be thus limited in most land 
yertebrates. Ido not at all understand why the lateral fold should not sometimes have resulted in the forma- 
tion of more than four limbs in Fishes. 
Sha 
