AQUATIC MAMMALS 



tral to a large proportion of present-day mammals. If this were the 

 case, then naturally there would be significant resemblances of the whales 

 to the ungulates as well as to the carnivores. But far more and far older 

 fossil material must be found before anything at all in this direction can 

 be proved. 



Modern whales are biserial as are the pinnipeds, and as in the case of 

 the latter it seems equally futile to argue as to whether the two groups 

 separated before or after they took to the water. Those who are but 

 casually acquainted with whales seldom realize! the fundamental dis- 

 tinctiveness of the baleen whales or Mystaceti and the toothed whales or 

 Odontoceti. The former, usually, but perhaps erroneously, believed to 

 be the most recently developed, are known no farther back than the 

 Oligocene, while a few remains of the latter have been detected in the 

 Upper Eocene. The differences that occur in the two groups may, how- 

 ever, mean nothing more than that their habits or environment differed 

 considerably at an early stage of their aquatic career, thus affecting their 

 rates of differentiation. For instance the ancestor of the baleen whales 

 may have lingered in rivers and shallow lakes, retaining considerable de- 

 pendence upon the land, while the toothed whale ancestor may at the 

 same time have taken to the sea and severed connection with the land at 

 an earlier time ; or vice versa. 



Gigantism in whales is an indication of overspecialization and could 

 be developed to the degree in which it now at times occurs only in an 

 aquatic mammal completely emancipated from the land. The elephant 

 seal, which may exceed 20 feet in length, seems now at the critical point 

 in this regard. Some increase in size or a slight increase in its aquatic 

 specialization would likely render it unable to leave the water, and we 

 do not know whether it would perish or survive as a result. 



Some whales descend to great depths, in the case of the cachalot pre- 

 sumably as far as a mile, where the pressure would be more than a ton 

 to the square inch, and it is purported to be able to submerge for con- 

 siderably more than an hour. These facts bring up a host of physiologic 

 questions which are extremely difficult of solution. 



[47] 



