12 MANUAL OF ZOOLOGY. 



Physiology treats of all the functions exercised by living 

 bodies, or by the various definite parts, or organs, of which 

 most animals are composed. All these functions come under 

 three heads : i. Functions of Nutrition, divisible into func- 

 tions of absorption and metamorphosis, comprising those func- 

 tions which are necessary for the growth and maintenance of 

 the organism. 2. Functions of Reproduction, whereby the per- 

 petuation of the species is secured. 3. Functions of Correlation, 

 comprising all those functions (such as sensation and voluntary 

 motion) by which the external world is brought into relation 

 with the organism, and the organism in turn reacts upon the 

 external world. 



Of these three, the functions of nutrition and reproduction 

 are often collectively called the functions of organic or vege- 

 tative life, as being common to animals and plants ; while the 

 functions of correlation are called the animal functions, as 

 being more especially characteristic of, though not peculiar to, 

 animals. 



6. DIFFERENCES BETWEEN DIFFERENT ANIMALS. 



All the innumerable differences which subsist between dif- 

 ferent animals may be classed under two heads, corresponding 

 to the two aspects of every living being, morphological and 

 physiological. One animal differs from another either morpho- 

 logically, in the fundamental points of its structure ; or physio- 

 logically, in the manner in which the vital functions of the 

 organism are discharged. These constitute the only modes in 

 which any one animal can differ from any other ; and they may 

 be considered respectively under the heads of Specialisation of 

 Function and Morphological type. 



a. Specialisation of Function. All animals alike, whatever 

 their structure may be, perform the three great physiological 

 functions ; that is to say, they all nourish themselves, repro- 

 duce their like, and have certain relations with the external 

 world. They differ from one another physiologically in the 

 manner in which these functions are performed. Indeed, it is 

 only in the functions of correlation that it is possible that there 

 should be any difference in the amount or perfection of the 

 function performed by the organism, since nutrition and repro- 

 duction, as far as their results are concerned, are essentially the 

 same in all animals. In the manner, however, in which the 

 same results are brought about, great differences are observable 

 in different animals. The nutrition of such a simple organism 

 as the Amoeba is, indeed, performed perfectly, as far as the 

 result to the animal itself is concerned as perfectly as in the 



