54 THE FUNCTIONS OF LIFE. 



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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 conti- 

 nually smaller, till it disappears altogether. We may often, 

 however, perceive some traces of its existence, but only in 

 a rudimental state, and a^ 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 arrangement of these similar portions either 

 round a central axis, or in a longitudinal series. The for- 

 mer is apparent in the verticillated organs of plants, and in 

 the radiated forms of zoophytes. The linear arrangement 

 is exhibited in the similar 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 ex- 

 hibits the lateral junction of similar but reversed structures. 

 The violations of this law are extremely rare; yet some re- 

 markable instances of anomalous formations, in this respect, 

 will hereafter be noticed. 



In treating of the particular functions of the animal and 

 vegetable economy I shall follow a different order from that 

 in which I have presented them in the preceding sketch. 

 As the Mechanical functions depend upon the simpler pro- " 

 perties of matter and the well known laws of Mechanism, I 

 think it best to commence with the examination of these. 

 Our attention will next be directed to the highly interesting 

 subjects which relate to the Nutritive or Vital functions 

 both of vegetable and animal structures: for as they involve 

 the chemical properties of organized substances, and are, 

 therefore, of a more refined and intricate nature than the 

 preceding, I conceive they will be best understood after the 

 general mechanism of the frame has been explained. These 



