INTRODUCTION. 15^ 



nicely balanced in the water, needs no support 

 from limbs as do terrestrial animals, neither are 

 the limbs needed to propel the body through 

 the water. Locomotion is effected in one of two 

 ways. Much elongated fishes, like sharks or eels, 

 for instance, move with great speed by rapid 

 undulations of the body. The forward motion is 

 effected by the pressure of the body against the 

 water, enclosed in the several incurved planes 

 arising from the un- 

 dulations. This un- 

 dulatory movement 

 is well expressed in ^ „ ^. ,. . , t^ . 



, ,. n a Fig .2.— Diagrammatic figure of an Eel, 



the diagram, ng. J. showing the nature of the undulatory 



Eelatively shorter movement of the body. 



fishes progress by powerful side to side move- 

 ments of the tail; and since the majority of 

 fishes seem to have shortened up the body, for 

 the sake of using the tail as a propeller, it 

 is probable that this is the more useful form 

 of movement of the two. 



If any doubt the reasonableness or probability 

 of the characteristic "fish-like" form having 

 arisen as a result of adaptation to the mechani- 

 cal needs of the environment, let him pause and 

 consider what has happened to certain aquatic 

 mammalia — to wit, the whales and porpoises. 

 These animals are so peculiarly fish-like in form 

 that they are very commonly regarded as fish. 

 The authorities at the Natural History depart- 

 ments of the British Museum are being con- 

 stantly appealed to, to settle arguments such as 

 whether or no the whale is a fish. The same 

 spindle-shaped tapering form of body, the pre- 



