THE r UNCriONS OF LIFE. 47 



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 purpose 

 foreign to its usual destination, or to qualify 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 auxiliary organs of progressive mo- 

 tion ; and in the Draco volans, or flying lizard, they 

 are extended outwards from the sides to serve as 

 wings. The teeth, usually intended for mastica- 

 tion, are in many animals enlarged in order to con- 

 stitute weapons of offence, as in the Elephant, the 

 Boar, the Narwhal, and the Pristls. In like man- 

 ner, in the Crustacea, organs of the same general 

 structure 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 dis- 

 tinction between them. 



In pursuing the ascending series of animal struc- 

 tures we meet also with instances of a contrary 

 change, yet still resulting from the continued ap- 

 plication of the same principle. An organ which 

 has served an important purpose in one animal, 

 may be of less use in another, occupying a higher 

 station in the scale, and the change of circum- 

 stances may even render it wholly useless. In such 

 cases we find that it is gradually discarded from 

 the system, becoming continually smaller, till it 



