AQUATIC MAMMALS 



be such a tendency it is often set at nought by diversity of function 

 of the two members. 



At any rate the few manual changes in those mammals mentioned 

 above are too sHght in degree either to be of much use to their possessors 

 or to be of much significance. They may or may not be indicative of 

 a hne of future progress. 



The development of the manus in the platypus is comparable in some 

 respects to that of turtles, and in others to the sea-lion. Apparently 

 there has been ample time since this queer beast took to the water for 

 it to experiment fully and to adopt in marked degree those peculiarities 

 best suited to its needs. Its body is definitely flattened and it is likely 

 that this always has been a characteristic. Its tail probably assumed its 

 present form not primarily as a rudder for swimming but to keep the 

 animal on the bottom while feeding. Hence in order to overcome 

 oblique movements which would be affected by the alternate kicking 

 of one pair of limbs it probably, throughout a very long period, swam 

 by diagonal kicks of all four feet. As a result it is likely that the 

 webbing of these four members progressed for a while at a fairly uni- 

 form rate. But it is the invariable rule among mammals that when 

 aquatic specialization has attained some considerable degree of perfection 

 the tendency is for a single member or pair of members to take over 

 the primary means of propulsion. If for no other reason than that the 

 fore limbs are better situated for acting in co-operation with the tail 

 for keeping the animal near the bottom it is likely these would receive 

 the greater stimulation for development. This might be either increased 

 still further, or else hindered, according to the quality of the action 

 involved, by the presumable function of removing the bottom debris 

 and stones in search for food and of digging burrows. At any rate 

 they have gained the evolutional ascendancy over the hind limbs. But 

 the future of this creature is hardly predictable. The manus may or 

 may not eventually assume the form of a flipper, or for that matter the 

 tail might ultimately take over the function of a primary propulsive 

 organ. 



It is seen that the manus of the platypus (fig. 42) is very highly 

 and peculiarly modified, the characters, in fact, being entirely unique. 

 These, however, are not satisfactorily brought out in Burrell's photo- 

 graphs. In a spirit specimen before me the membrane extends con- 

 siderably beyond the nails, but projecting even farther are leathery 

 thickenings of the membrane. There is but one of these to the first 

 and fifth digits, extending in continuation of the digit. Each of the 

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