466 University of California Publications in Geology [Vol.7 



Eibs 486 



Pelvis 486 



Kelation of Characters of Pelvis to Aquatic Adaptation 487 



Fore Limbs 488 



Hind Limbs 489 



Stage of Evolution of Latax 491 



Palaeontologic Evidence Bearing on Problem of Origin of Sea-otter.... 492 



Variation in Latax 493 



Summary 495 



INTKODUCTION 



Adaptation to life in the water presents many possibilities 

 in connection with studies of variation of form, since the influ- 

 ence of the factor of environment is so nearly a constant, and 

 the nature and direction of the variation may be clearly seen. 

 This is especially true of animals which have been adapted to life 

 on land, and have later entered upon an aquatic existence. In 

 such cases one should be able, from a consideration of their 

 present form and palaeontologic history, to follow the course of 

 evolution with more than usual certainty. Under these cir- 

 cumstances, when the nature and significance of the changes 

 are so clearly shown, there should also appear to be a greater 

 probability of gaining some insight into the process by which 

 these changes are effected. Unfortunately, this ideal is in few 

 if any cases attainable, partly because ancestry often can not be 

 traced, owing to lack of the necessary palaeontologic material. 



The aquatic Mammalia, the Cetacea, Sirenia and Pinnipedia, 

 are groups already far advanced in adaptation. Since they are at 

 present almost exclusively marine, they offer an especially wide 

 field for investigation of this kind. Work upon these highly special- 

 ized groups, however, does not to any extent reveal data on the 

 beginning stages of adaptation to aquatic life, and the early 

 structural changes are, in many respects, the most significant, 

 especially from the point of view of an enquirer into the nature 

 of the processes involved. 



A study of the sea-otter (Latax lutris) might presumably 

 be of value in this connection, since the animal is one exhibit- 

 ing variations which are probably, in some degree, analogous to 

 those shown by the ancestors of the groups above mentioned not 



