56 THE FUNCTIONS OF LIFE. 



its relation to other species belonging to the 

 same type, and demonstrates the existence of a 

 general plan in their formation. I shall have 

 occasion to mention several striking instances 

 of this kind, both in the animal and vegetable 

 kingdom. 



In following the transitions from one model of 

 structure to another, we often observe that a par- 

 ticular organ has been very greatly enlarged, or 

 otherwise modified to suit some particular pur- 

 pose, foreign to its usual destination, or to qua- 

 lify it for performing some new office, rendered 

 necessary by the particular circumstances in 

 which the animal is placed. Thus the ribs, 

 which in quadrupeds are usually employed for 

 respiration, are in serpents converted into auxi- 

 liary organs of progressive motion : and in the 

 Draco volans, or flying lizard, they are extended 

 outwards from the sides to serve as wings. The 

 teeth, usually intended for mastication, are in 

 many animals enlarged in order to serve as wea- 

 pons of offence, as in the Elephant, the Boar, 

 the Narwal, and the Pristis. In like manner in 

 the Crustacea, organs of the same general struc- 

 ture are converted sometimes into jaws, some- 

 times into feelers, (or palpi) and sometimes into 

 feet ; and the transition from the one to the 

 other is so gradual that it is difficult to draw a 

 proper distinction between them. 



In pursuing the ascending series of animal 

 structures we meet also with instances of a con- 



