INTRODUCTION 



aquatic adaptation whether he inhabit large rivers or coastal waters. 

 And heretofore all steps in his development have presumably been by 

 slow stages. He himself soon has no further need for the land, for 

 although he still likes to bask on a rock in the sun, it is not long before 

 he can forego this luxury, does such a course seem expedient. But 

 his young must pass their early life upon the land and if this necessity 

 persists he can never become exclusively aquatic. 



The latter is a fundamental factor in the evolution of a highly de- 

 veloped aquatic mammal — the requirements of its young. The newborn 

 seal and sea-lion will drown if forced into the water. If the terrestrial 

 or amphibious enemies of pinnipeds should multiply sufficiently to 

 destroy more than the critical number of young needed to replace the 

 breeding stock this order would become extinct, for they are not yet 

 ready to desert the land entirely. In a few million years they may be 

 ready to do so. At that time, if driven from their rookeries, they may 

 have become sufficiently modified so that enough young might survive 

 an aquatic birth for the perpetuation of the race. The whales and 

 sirenians have successfully taken this step and forever severed their 

 slightest connection with dry land. 



The development of existing aquatic herbivores was probably con- 

 siderably different, for this sort of mammal is under no necessity for 

 rapid movement in order to secure sustenance. From a palustral habi- 

 tat the ancestors of the sirenians doubtless subsided sluggishly into deep 

 water that was free from large enemies, chiefly for the purpose of es- 

 caping troublesome terrestrial carnivores, and have since been under the 

 necessity of doing little but move from one submerged pasture to an- 

 other. 



All the above resolves into the simple statement, first advanced by 

 Kukenthal (1890) that the degree of aquatic specialization in mammals 

 is corollary to the amount of connection retained with the land. 



[7] 



