PREFACE. xi 



parts, conducive to the fulfilment of a pre-ordained purpose and 

 a final aim : these opposite yet reciprocally complemental factors 

 co-operating to the ultimate result, with different degrees of dis- 

 turbance, yet without destruction, of the evidences of the typical 

 unity. 1 



Thus, the dentition of Vertebrates will be seen to pass from 

 irrelative sameness and multitude to the state in which the teeth 

 in the same jaw are classed according to diversities of form and 

 function, and where each tooth has its own character, bears its 

 own name and symbol. 



In like manner may be traced the gradations by which the 

 terminal divisions of the limb ascend from the multitude of many 

 jointed rays, swathed in a common sheath of integument, to indi- 

 vidual freedom, with reduction of number and of joints, and with 

 a special form and action ; according to which each digit in the 

 human hand, e.g., has its special name and symbol, and can be 

 combined in action with any other digit for a particular purpose. 

 The same principle, through reduction of number with differen- 

 cing of the parts, is exemplified by the fact that the competent 

 anthropotomist will distinguish and name each of the four and 

 twenty ' true vertebra' of the human skeleton. 



In the Mammalian class there are four muscular pulsatile 

 cavities concerned in the propulsion of blood ; but they differ from 

 those cavities in the Annelide, in each having its own special 

 structure and powers, and being in such relation with another 

 cavity that the whole can combine to effect two complete but 

 mutually related systems of circulation, the four pulsatile cavities 

 constituting one complex and perfect ' heart.' The ox has four 

 bags for the digestion of food ; but they differ from those cavities 

 of the Polyyastria, not only in their minor number and more de- 

 finite structure as bags, but by each performing a distinct part in 

 the process of digestion, and combining with the rest, in mutual 



1 This idea will be found more fully exemplified in my work, ' Principes d'Osteolo- 

 gie Comparee,' Sro. (Paris) 1855, p. 366, et seq. 



