TRANSFORMATIONS. 153 



various organs of the body. The present will 

 indicate a few of the changes of another kind, of 

 which evidence is to be found in a study of the 

 anatomy of fishes. These well show how the 

 already elaborated structures and secretions may 

 combine to form yet other structures. Often 

 these arise in parts of the body which have been 

 relieved of their original functions, and are there- 

 fore free to undertake such new duties as may be 

 beneficial for the continuance of the species. 

 This replacement of one organ by another is 

 known as the substitution of organs. Instances of 

 such substitutes we shall discuss here, together 

 with cases wherein organs of long standing have 

 become further adapted to perform new duties 

 without undergoing any great changes in external 

 appearance. 



A simple instance of the substitution of organs 

 is illustrated by certain members of the skate 

 tribe. The skates are nothing more than highly 

 specialised sharks. They have become skates, 

 we may put it, by virtue of the fact that they 

 have transferred the seat of locomotion from the 

 tail to the pectoral fins. These have become 

 enormously developed in consequence — a de- 

 velopment accomplished at the expense of the 

 tail, which has become greatly reduced in size, 

 and functions only as a rudder. The changes in 

 the general form of the body, consequent on this 

 substitution of the pectoral fins for the tail, have 

 become so marked that naturalists once separated 

 all the animals so affected into a group by them- 

 selves — the skates. The release of the tail from its 

 original function of propelling has been followed 



