154 Dr. W. Kiikenthal on the Adaptation of 



the Yellow-bellied Water-Mouse (Hydromys chryso- 



gaster) ; 

 Holochilus ; 



the Water- Vole (Arvicola amphibius) ; 

 the Musk-Rat (Fiber zibethicus) ; 

 the Beaver (Castor fiber} ; and 

 the Capybara (Ilydrochoerus capybara). 



Among the Insectivora : 



the Water-Shrew (Sorex fodiens) and 

 the Desman (Myogale). 



Among the Ungulata (Artiodactyla) : 



the Hippopotamus (Hippopotamus amphibius) . 



Among the Carnivora : 



the Otter (Lutra) and 

 the Sea-Otter (Enhydris). 



On a closer examination of these more or less exclusively 

 aquatic creatures we find that, notwithstanding the variety of 

 orders to which they belong, they nevertheless possess many 

 common structural features, and we shall be perfectly safe in 

 ascribing these points of agreement to the influence of the 

 aquatic life. 



Owing to adaptation to the life in water, therefore, changes 

 have taken place in the organization of these animals. The 

 extent of these changes will vary in direct ratio with the time 

 during which the influence of the water has been operating ; 

 or, in other words, the changes will be greater or less according 

 as a longer or shorter time has elapsed since the animals in 

 question exchanged the terrestrial for the aquatic life. We 

 have further to consider that many of the aquatic mammals 

 which we have enumerated live by no means entirely in the 

 water, but are at times land-dwellers as well. The next 

 point to be ascertained is whether these changes are always 

 manifested in the same direction. 



In the case of a large number of forms, in order to become 

 acquainted with the changes which have taken place, it is 

 sufficient to compare the animals with their nearest relations 

 on land. But, as a matter of course, closely allied terrestrial 

 forms are to be found in the case of those mammals alone 

 which have only recently adopted the water as their element ; 

 those which have been aquatic for a long time will be able to 

 show no relations on land, since both the water as well as the 

 land branch have struck cfT from one another in diverging 



