32 CLASSIFICATION AND ADAPTATION 



respiratory movements, and consequently there was 

 a resultant negative pressure beneath the body. 

 By means of a model made of a thin flexible 

 sheet of rubber, at each end of which on one side 

 was fastened a short piece of glass tube, I was 

 able to imitate the physical action observed in the 

 fish. A long piece of rubber tube was attached to 

 one of the pieces of glass tube, and brought over the 

 edge of the glass front of an aquarium. The long 

 rubber tube was set in action as a siphon and the 

 sheet of rubber placed against the glass. As long 

 as water was running through the siphon the sheet 

 of rubber remained pressed against the glass and 

 supported. As soon as the current of water was 

 stopped the apparatus fell to the bottom of the 

 tank. In this model water passed out from beneath 

 the rubber through the glass tube attached to the 

 siphon and passed in by the opposite glass tube, 

 and at the sides of it. The latter tube represented 

 the respiratory channel of the fish, and the space 

 between tube and rubber represented the spaces 

 between the head of the fish and the vertical surface 

 to which it clung. 



In the fish the marginal fins not only extend to the 

 base of the tail, but are broader at the posterior 

 end than elsewhere, whereas in other Flat-fishes 

 the posterior part of the marginal fins are the 

 narrowest parts. The shape of the fins and the 

 breadth of the body posteriorly, then, are adaptations 

 which have a definite function, that of enabling 

 the fish to adhere to vertical surfaces. But, on the 

 other hand, the extension of the marginal fins in a 

 transverse direction beneath the tail has no use in 

 the process of adhesion, nor has any other use been 



