40 ANIMAL LIFE PAST AND PRESENT. 



which it is a primitive one, is afforded by the different 

 modes in which aquatic animals breathe. Thus in 

 fishes the air necessary to oxygenate the blood is 

 obtained from that dissolved in the water itself by its 

 constant passage over those peculiar comb -like organs, 

 highly charged with blood, known as gills; such 

 animals having, therefore, no occasion to come to the 

 surface of the water to breathe. In other animals, 

 however, such as Whales and Grampus (Fig. 14), 

 atmospheric air is breathed directly by means of lungs, 

 necessitating visits at longer or shorter intervals to the 

 surface, and it is in such instances that we may safely 

 infer that the adaptation to an aquatic life has been 

 gradually developed from ancestors whose normal 

 habits were terrestrial, since otherwise the gills would 

 never have been lost. That animals whose original 

 mode of life was a purely aquatic one have tended in 

 some cases to assume a terrestrial existence is proved 

 by the case of the Common Frog, which commences 

 life as a gill- breathing, swimming creature, to all 

 intents and purposes a fish, and ends by being an 

 air-breathing reptile, as much at home on land as in 

 the water, although retaining the power of swimming. 

 On the other hand, the Seals and Otters show us how 

 an originally terrestrial type of animal has become 

 adapted to pass a large part of its time in the water, 

 which has become its natural element. 



The term " Swimming Animals " is, of course, a very 

 wide one, since a considerable proportion of animals 

 whose normal habits are terrestrial can, on occasion, 



