INTRODUCTION. 11 



same mould. Earth, air, and water are all 

 peopled, and the inhabitants of each of these 

 three worlds frequently bear very close resem- 

 blances one to another without being in the least 

 degree related. That is to say, there is evidence 

 of conformity to the mechanical needs of the 

 environment, resulting in a superficial conformity 

 in external appearance. 



The fishes afford us an admirable object lesson. 

 They are creatures fashioned by a series of 

 gradual improvements to dwell in water. To 

 move freely in this comparatively dense medium 

 entails conformity to certain mechanical needs. 

 This conformity has resulted in the characteristic 

 "fish-like" form: the compressed head and 

 trunk, tapering gradually to the tail, giving the 

 whole the form of a rounded wedge; to this 

 wedge-shaped body keels have been added along 

 the back and along the belly, as well as balancing 

 and steering organs projecting from the sides. 

 These keels and balancers we call the fins, and 

 because we shall have to make frequent reference 

 to these fins it would be well to pause here to 

 study their arrangement and distinctive names 

 in the accompanying picture (fig. 1). The fins, 

 which we have likened to keels, are known as the 

 median fins, whilst those which act as balancers 

 and for steering, are known as the paired fins. 

 These are never more than four in number, and 

 correspond with the arms and legs of terrestrial 

 animals. But the fish is, so to speak, balanced in 

 the water, and needs no support from the limbs, 

 which owe their peculiar form to adaptation to 

 their special requirements. The terrestrial limbs 



