THE FUNCTIONS OF LIFE. 5SL 



extent, as to render it wholly useless in that particular spe- 

 cies, although in a higher race of animals it fully performs 

 its proper function. Thus, we shall find that rudiments of 

 feet are contained within the bodies of various kinds of ser- 

 pents, which can obviously not be serviceable as organs of 

 progression. In the young of the whale, before its birth, 

 there is found in the lower jaw, a row of small teeth, which 

 do not rise above the gums, and can, therefore, be of no use 

 as instruments of mastication. Their farther growth is ar- 

 rested, and they are afterwards obliterated. This imperfect 

 or rudimentary condition of an organ indicates its relation 

 to other species belonging to the same type, and demon- 

 strates 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 followino; the transitions from one model of structure 

 to another, we often observe that a particular organ has been 

 very greatly enlarged, or otherwise modified to suit some 

 particular purpose, 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 aux- 

 iliary organs of progressive motion: and in the Draco vo- 

 lans, 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 weapons of offence, as in the Elephant, the Boar, the 

 Narwal and the Pristis. In like manner, in the Crustacea, 

 organs of the same general structure are converted some- 

 times into jaws, sometimes into feelers, (or palpi,) and some- 

 times 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 contrary change, yet still re- 

 sulting from the continued application of the same principle. 



