CHAPTER III. 



SWIMMING ANIMALS. 



IN the preceding chapter we discussed the various 

 structural modifications by means of which the 

 members of different groups of animals are enabled 

 to fly, or, in other words, to swim in the aerial 

 ocean. From the observations there recorded, it 

 is evident that all the creatures adapted for this 

 peculiar mode of life have been specially modified 

 for that purpose ; flight thus always being a power 

 which has been specially acquired, and not one 

 which was an original attribute of any group of 

 animals. 



It is our purpose in the present essay to notice in 

 a somewhat similar manner the various adaptations of 

 the structure of certain animals whereby they are en- 

 abled to swim in the denser medium of water. And 

 here we shall find that while there is conclusive 

 evidence to show that in many instances this power is 

 an acquired one, yet there are others which lead to 

 the belief that in certain groups it is a primitive 

 function. Some clue as to the groups in which this 

 power of swimming is an acquired one, and those in 



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