INTRODUCTION 



All aquatic mammals have, of course, evolved from terrestrial ones, 

 although paleontology does not furnish us with evidence regarding 

 the early stages of any aquatic genus, but many of the steps may be 

 reconstructed with a feeling of considerable assurance. 



The surf is usually too boisterous to appeal to a small or medium- 

 sized mammal with a desire for an occasional swim, and shore waters 

 offer too few places for concealment. Large carnivores might take 

 directly to the sea, as the polar bear is doing, but it is probable that 

 most aquatic mammals first ventured into fresh water, the smaller sorts 

 into bogs and small streams, and the larger into swamps or rivers. 

 Those of carnivorous (including insectivorous) propensities took this 

 step almost invariably in search of prey which seemed to them easier 

 of capture than terrestrial food, or else more palatable. At this stage 

 carnivores probably would not seek the water as a means of escape from 

 their enemies and their swimming must have been only superficial, 

 consisting of a plunge after a fish or a short sally across a stream. 

 An aquatic herbivore probably would start its career either by seeking 

 the shelter of densely grown swamps as a diurnal refuge from its 

 enemies, to secure aquatic plants which attracted it, or to escape the 

 torment of insects by standing in the water. It would not take long, 

 geologically speaking, for either the carnivore or herbivore to discover 

 that the water offered a safe refuge from most of its enemies. But this 

 must also be taken into consideration: an abundance of aquatic enemies, 

 such as crocodiles, would tend to discourage the adoption of an aquatic 

 existence and just this factor may have prevented the aquatic develop- 

 ment of many a promising mammalian stock. 



A mammal with somewhat palustrine tendencies might, therefore, 

 likely have been forced into the water either by similar forms that 

 competed with it for terrestrial food, or by enemies which pursued it 

 perhaps both. If in the water it encountered an abundance of enemies 

 in the shape of large fish or hungry reptiles it might either be exter- 

 minated, forced back to face terrestrial enemies as being the lesser 

 of two evils, or obliged to content itself with a borderline existence, 

 foraging in shallow water amid protective vegetation. This latter 

 has constituted the fate of most small, semi-aquatic insectivores and 

 rodents. Some of them may eventually encounter an environment 

 sufficiently favorable so that they can relinquish such a game of hide- 

 and-seek and boldly take to the open water, but most of them will 

 not, and the latter can never develop very highly specialized aquatic 

 modifications. 



[5] 



