224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



demanded for walking. Not unless, and until, they were compelled 

 to live permanently in deep water would the limbs undergo "adjust- 

 ment" to swimming. 



The sea lion shows us an advanced stage in the transformation of 

 walking — ^into swimming legs. Though it still passes much time 

 out of the water, all its food had to be obtained in the sea. Hence, 

 intensive use of the fore limbs has slowly converted them into 

 "flippers" differing from those of the Cetacea in that they still show 

 digits externally, terminating in claws. But tlie hind limbs are also 

 becoming greatly modified, so much so that they can do no more than 

 enable the animal to "hobble" along, with the aid of the flippers, 

 when on land. The seal carries us a stage further, for the hind 

 limbs can no longer be turned forward to contribute to the support 

 of the body when ashore. They are directed permanently backward, 

 with their plantar surfaces apposed. The "flipper" in the case of 

 the seals is never elongated after the fashion of the sea lion, and 

 we must attribute this fact partly to the "intensity" of its swimming, 

 and partly to the fact that here, as elsewhere, the response has been 

 different because the "qualities" of the tissues forming the limb are 

 different. 



In the porpoise and the rest of the Cetacea, we find what we may 

 call the "logical sequence" of a still more intensively aquatic life, 

 which has profoundly affected the whole body. External hind limbs 

 have now vanished, and the fore limbs have not merely become trans- 

 formed into flippers but their internal structure has undergone a 

 drastic change to which reference must be made again. The fishlike 

 mode of life has brought about, with a few most interesting and 

 important exceptions, a "dorsal fin", while the tail has developed 

 horizontally directed "flukes", which have a very instructive bearing 

 on this matter of "adjustment", as may be seen on plate 2. The 

 Cetacea, being lung-breathers, must come periodically to the surface 

 to breathe. The horizontal tail has been formed by the response 

 of a once otterlike, cylindrical tail to the resistance of the water in 

 using the tail to drive the body downward after food and upward 

 for air, for these movements gradually started a flattening out of 

 the tail, and its expansion into flukes. The fish, which has no need 

 to come incessantly to the surface, drives the tail in a lateral direc- 

 tion, to impel the body forward, hence the tail fin is vertical. The 

 manatee, be it noted, is in no way related to the Cetacea, yet it has 

 assumed a similar form. The fore limbs have become flippers, 

 there are no external hind limbs, and there are tail flukes as in the 

 Cetacea. Moreover, in the river dwellers these flukes are spatulate 

 in shape, but in the marine Steller's sea cow, now extinct, and the 

 dugong {Halicore), they have the triangular form of the Cetacea. 



