THE PECTORAL LIMB 



Whether for propulsion or as a speciahzed rudder the anterior hmb 

 of an exclusively marine mammal may be expected to have become rela- 

 tively stiff, although elastic. In effect it becomes either a horizontal rud- 

 der or an oar. As such it should have a single joint, analogous to a 

 row-lock, and this should be situated just within the body contour. There 

 should not be additional joints and those already existing will tend to be- 

 come immobile, unless there be need for them because of feeding habits. 



In attempting to determine the ideal position for the flipper of a ma- 

 rine mammal that uses this member for steering there are encountered 

 a number of interacting factors which render the question a difficult one. 

 It is apparent that in the whale the most effective position for simple 

 steering fins would be in the vertical plane, one above and another be- 

 low, so that by slight tilting the body would be thrown to the right or 

 left: but such an alteration in fore limb posture would be impossible. 

 The same act could be accomplished by carrying the limb horizontal but 

 tilting it so as to offer as much resistance to the water as possible and 

 at the same time pressing the opposite limb against the body. This 

 concerns only rudder action. But some equipment for pure equilibration 

 should also be advantageous. If a whale swim but languidly with only 

 the tail proper involved then this may not be necessary, but if it swim so 

 violently that there is no real interval between the cephalic and caudal 

 amphikinetic parts, there will be a greater or lesser tendency for the 

 thorax to move upward or downward at each stroke of the flukes. In 

 order to reduce this lost motion to a minimum, large flippers widely ex- 

 tended would be of the greatest aid. In addition, an animal provided 

 with flippers of this character so held would find it easiest to depress or 

 elevate the head by slight tilts of these members. Thus it is seen that 

 in an animal such as a whale a pair of horizontal flippers should best ac- 

 complish elevation and depression of the body (equilibration proper) 

 and by operating one at a time these might be just as efficient in steer- 

 ing from side to side as if they were vertically placed. 



The above argument sounds logical but it may be complicated to a 

 large degree by yet another factor. Back in the early history of the 

 world when the first vertebrates were being evolved these developed an 

 equipment of primitive fin folds. Opinion differs as to the ideal arrange- 

 ment of these and there are proponents of two different theories, but all 

 are agreed that there was a pair situated latero-ventrally from which 

 were derived all four vertebrate limbs. The point which is of interest 

 in the present connection is that there was evidently an elemental stimu- 

 lus connected with aquatic requirements which operated to develop pec- 



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