AQUATIC MAMMALS 



suspected. Of these three circumstances it seems that the second would 

 be more critical in possibly causing a flat tail, but I do not feel assured 

 that this is the case and I prefer to make it plain that I have no conviction 

 on the subject. 



The Pinnipedia may be said to lack a functional tail and it seems 

 highly probable that they have descended from an ancestry with tail 

 much as in the living bears. In the walrus the "crotch" stretches un- 

 interruptedly between the heels caudad of the bony tip of the tail. In 

 a sea-lion {Zalophus) the external tail constituted but six per cent 

 (60 mm.) of the total length, while in a seal (Pboca) this percentage 

 was seven (72 mm.). The tail can have not the slightest use as an 

 active aid either for propulsion or steering, but in the sea-lion the tail 

 was slightly thicker in horizontal than in vertical dimension, while in 

 the seal this was very much more pronounced. In the latter at least 

 it was plainly to be seen that the shape of the tail permitted this member 

 to fit perfectly into the cleft between the heels so as to effect an uninter- 

 rupted body contour. The stimulus for this specialization is unknown. 

 It was probably of an entirely different nature from that which has 

 caused a broadening of the tail in aquatic forms in which this member 

 is the primary propulsive organ, and for the present all that can be said 

 is that it constitutes another instance of the fact that when an animal 

 experiences the need for a modification, the latter will often appear even- 

 tually. 



The tail of the otter is of particular interest in the present connection 

 for the reason that it seems to me likely that the original ancestors 

 of the whales had a largely similar conformation of the body. The tail 

 of this animal is exceedingly thick at base, so that there is a gradual 

 taper from the hindquarters to the tail tip. At the same time the body 

 is rather long and sinuous and the legs are short. The feet are often 

 used in swimming, but as propulsive organs they seem to be of decidedly 

 secondary importance. Swimming is mainly accomplished by curvature of 

 both the lumbar and caudal regions in the vertical plane. Extension of 

 the vertebrae, after flexion, is often accompanied by a kick of both hind 

 legs, not in unison nor yet in alternation, but with a sort of galloping 

 action, but the animal appears able to swim with equal ease and speed 

 when the feet are folded against the body. The shape of both body 

 and tail base, together with great muscular power in the latter region, 

 is largely accountable for this, and as the feet are relatively small and 

 little changed for swimming (in the common Lutra), it seems either 

 that the feet of mustelids have been unusually resistent to modification 



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