INTRODUCTION. XIX 



are acquainted lias the power of maintaining its 

 existence and increasing in size ; it likewise exe- 

 cutes perceptible movements, and gives rise to 

 other beings similar to itself. So also is it with 

 the most highly organised animal. What then is 

 the natm-e of the physiological distinction between 

 both? 



In the simple animal being to which we have 

 alluded it can scarcely be said that distinct organs 

 exist, all parts of the body appearing to possess 

 the power of performing, when called upon, any 

 one of the necessary functions. In other animals, 

 we recognise the presence of separate instruments 

 for the discharge of each of the three great groups 

 of functions above enumerated. In others, again, 

 we find that one or more of these is resolvable 

 into what may be termed " secondary functions,'' 

 to each of which a special organ has been allotted. 

 Finally, we meet with animals in which this "spe- 

 cialisation " of the functions and multiplication of 

 dissimilar parts reaches its utmost extent. When 

 such is the case, it is not difficult to show that the 

 various vital actions are accomplished with a de- 

 gree of completeness and efficiency not observable 

 in those humble organisms whose position is at 

 the base of the animal scale. All this is said to 

 be in accordance with the principle of the physio- 

 logical division of labour. 



In the higher animals the nutritive processes 



are subdivided into functions of prehension, mas- 



a 2 



