THE FUNCTION AND STRUCTURE OF FINS 455 



next figure (fig. 13) represents a condition approximating to 

 that which is found in some primitive sharks and suggests a 

 way in which this change might have been brought about, 

 viz. by a rocking outwards of a portion of the basal skeleton. 



The fin has now a considerable portion free for use in 

 connection with the new or lateral rudder function. Meanwhile 

 in order to provide this movable fin with a firm basis to 

 work upon, the front part of the basal skeleton seems to have 

 become modified into a limb girdle by extending upwards and 

 downwards in the body wall. Some of the rays still worked 

 upon this girdle, the remainder worked with the free portion 

 of the basal skeleton which had rocked outwards. This latter 

 could still be used as a keel but was concerned mainly with 

 the performance of the new function. The former still served 

 mainly as a keel but became available also for tilting by reason 

 of another change exemplified in the sturgeon. Up to this 

 point in our inquiries the line of attachment of the front rays 

 to the shoulder girdle — which may be called the glenoid line — 

 has been parallel to the length of the fish. In the sturgeon 

 (fig. 14) the girdle has rotated so that this line is nearly at 

 right angles to the axis of the fish. This rotation has made 

 it possible, not only for the hinder margin or half, but for the 

 whole fin to be tilted like a lateral rudder. In the sturgeon 

 the joint along this glenoid line is so stiff that the fins can 

 be no more than tilted. A less clumsy joint would have 

 enabled it to carry the tilting to such an extreme that the plane 

 of the fin would have been at right angles to the axis of the 

 fish. The brake function would then have been attained. 



The rotation of the girdle with the glenoid line naturally 

 brought the front rays into conflict with the hinder rays and 

 the latter became crowded between them and the free basal 

 skeleton. The sequel to this crowding is their complete dis- 

 appearance in the higher bony fishes in which only the rays 

 which are attached to the girdle are present (fig. 15), The 

 intermediate stages in this process are preserved for us in the 

 fins of lepidosteus and amia. 



The loss of these rays apparently under the stress of 

 mechanical necessity has its parallel in the wing of the bird. 

 The ancestor of the bird undoubtedly had five digits, now 

 there are only three, the two outer ones having disappeared. 

 This is associated with the way in which the wrist joint of the 



