THE FUNCTIONS OF LIFE. 57 



trary change, yet still resulting from the conti- 

 nued application 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 circumstances may even render it 

 wholly useless. In such cases we find that it is 

 gradually discarded from the system, becoming 

 continually smaller, till it disappears altogether. 

 We may often, however, perceive some traces 

 of its existence, but only in a rudimental state, 

 and as if ready to be developed, when the 

 occasion may demand it. 



In the greater number of organic structures 

 we may trace a tendency to the repetition of 

 certain organs, or parts, and the regular arrange- 

 ment of these similar portions either round a 

 central axis, or in a longitudinal series. The 

 former is apparent in the verticillated organs of 

 plants, and in the radiated forms of zoophytes. 

 The linear arrangement is exhibited in the si- 

 milar segments of annulose and other articulated 

 animals, and also in the pieces which compose 

 the spinal column of vertebrated animals. In 

 these two latter classes, also, a remarkable law 

 of symmetry obtains in the formation of the two 

 sides of the body, which exhibits the lateral 

 junction of similar but reversed structures. The 

 violations of this law are extremely rare ; yet 

 some remarkable instances of anomalous forma- 

 tions in this respect will hereafter be noticed. 



