398 THE MECHANICAL FUNCTIONS. 



difficulty identified : on some occasions, as we 

 have just seen in the spinous bones of fishes, its 

 accessory structures are muUiplied, as if conti- 

 nued efforts were made by the system to repeat 

 the same structures. Amidst all these modifica- 

 tions, the parts that preserve the greatest con- 

 stancy of form are those which are of most im- 

 portance, and which are constituent parts of the 

 primordial type of the class to Avhich the indivi- 

 dual animal belongs. 



The spinal column is generally prolonged at 

 its posterior extremity into a series of vertebras, 

 which are sometimes exceedingly numerous ; 

 decreasing in their size as they extend backwards, 

 and having continually smaller processes, the 

 one disappearing after the other, till all of them 

 are lost, and nothing remains in those at the 

 extremity of the series but the cylindrical bodies 

 of the vertebrae. Even these become stinted in 

 their growth and ossification, until we find 

 the terminal pieces generally remaining in the 

 state of cartilage. Such is the structure of the 

 osseous support of the tail, as seen in many 

 quadrupeds in its most developed forms. It 

 illustrates the law, that when in any system 

 there occurs a frequent repetition of the same 

 structure, the evolution, in the latest of those 

 repetitions, becomes less perfect, and ends by 

 being abortive. In the present instance, the 

 consequences of this law are highly advanta- 



