THE PELVIC LIMB 



limbs, should have an equilibrator or rudder for vertical steering at the 

 hinder end. In the mammal the tail is of such form that it should be able 

 to accomplish this unaided, while in the turtle, without an effective tail, 

 the hind feet are specialized for this purpose. 



Attention may here be called to conditions in some of the extinct 

 aquatic reptiles. In no highly modified reptile of this sort do the 

 hmd limbs seem to have disappeared completely, although in the most 

 specialized sorts, as Ichthyosaurus, they are considerably smaller than 

 the anterior ones. In those reptiles in which the transition of the tail 

 into a swimming organ was only partial, however, the hind limbs were 

 practically as large as the forward pair. In some of the more primitive 

 ichthyosaurs, for instance, such as Cymbospondylus, the body was more 

 anguilliform, there was certainly no broadly spreading caudal fins, and 

 all four paddles were about equally well developed. These details are 

 suggestive that not only were the inherent variational capabilities of the 

 reptiles diflferent from mammals, as already claimed, but that most of 

 these large aquatic reptiles were descended from broad-beamed ances- 

 tors, which obliged swimming after the mud-turtle method, somewhat 

 like the case of the hippopotamus, for long ages. But the ichthyosaur 

 of the more familiar type hardly has need for two sets of equilibrators 

 in the same plane and the posterior feet were evidently in course of 

 elimination when these creatures became extinct. 



Perhaps there are some who will question my conviction that in air 

 breathing mammals, and presumably in reptiles, but one pair of equilibra- 

 tors is necessary in the same plane, for they may recall that in many 

 fish there are all sorts of accessory fins in a variety of situations. But 

 this seems to be an entirely difi^erent matter. The equilibration of small 

 fish is a much more delicate matter, and fish have precision apparatus for 

 remaining in one exact position in spite of currents and other disturbing 

 influences. In larger forms of difi^erent structure fins must be much more 

 substantial (i.e. thicker), and furthermore, an air breathing vertebrate 

 cannot spend an indefinite amount of time drowsing in one spot or 

 threading its way delicately in and out of submarine growth deep be- 

 neath the surface. Even in the sharks, as in some other large fish, there 

 is the same tendency as exhibited in cetaceans and ichthyosaurs for the 

 elimination of accessory fins and the development of a single pair of 

 equilibrating paddles, in addition to a' dorsal fin. The tendency in large 

 aquatic forms thus appears to be for the acquisition of three equilibrating 

 devices spaced at more or less regular intervals. 



[273] 



