XVlll INTRODUCTION. 



Under the title of the " Balancing of Organs " 

 some morphologists have sought to establish a 

 principle which may lead to the adoption of erro- 

 neous conclusions. This principle, rightly stated, 

 is as follows : — The excessive development of 

 one organ is often accompanied by a proportional 

 deficiency in some other organ connected with it. 

 But to this rule there are many exceptions, and 

 in no case can it be proved that the atrophy or 

 deficiency of one organ is the result of the extra- 

 ordinary development of the other. 



The several functions, or vital actions, of ani- 

 mals may be conveniently arranged under three 

 groups, namely : — 



1. Those which are subservient to the growth 

 and maintenance of the organism — Functions of 

 Nutrition. 



2. Those which have reference to the continu- 

 ance of the species — Functions of Re/production. 



3. Those which enable the animal to perform 

 movements and become conscious of external im- 

 pressions — Functions of Relation. 



Of these, the Functions of Nutrition and Ee- 

 production, being common to all organised beings, 

 are sometimes designated " Functions of Ortranic 

 or Vegetative Life," while the Functions of Rela- 

 tion are distinguished by the title of ''Animal 

 Functions." 



The lowest form of animal life with which we 



