DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 27 



of minute or microscopical tissues (Gr. histos^ a web ; logos 

 a discourse). 



Physiology treats of all the functions exercised by living 

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

 most living beings are composed. All these various func- 

 tions, however, may be considered under three heads : — 

 I. Functions of Nutrition, divisible into functions of Absorp- 

 tion and Metamorphosis, and comprising all those functions 

 by which an organism is enabled to live, grow, and main- 

 tain its existence as an individual. 2. Functions of Repro- 

 duction, comprising all those functions whereby fresh indi- 

 viduals are produced and the perpetuation of the species is 

 secured. 3. Functiofis of Relation or Correlation, compris- 

 ing all those functions (such as sensation and voluntary 

 motion) whereby the outer world is brought into relation 

 with the organism, and the organism in turn is enabled to 

 act upon the outer world. 



Of these three, the functions of nutrition and reproduc- 

 tion are often spoken of collectively as the " organic " or 

 " vegetative " functions, as being essential to bare existence, 

 and as being common to animals and plants alike. The 

 functions of relation, again, are often spoken of as the 

 " animal " functions as being most highly developed in 

 animals. These functions, however, though more highly 

 characteristic of, animals, are not peculiar to them, but are 

 manifested to a greater or less extent by various plants. 



All the innumerable differences which subsist between 

 different organisms may be classed under two heads — 

 morphological and physiological — corresponding with the 

 two aspects of every living being. One organism differs 

 from another either morphologically, in the fundamental 

 points of its structure and the plan upon which it is built, 

 ox physiologically, in performing a different amount of vital 

 work, in a different manner, or with different instruments, 

 or both morphologically and physiologically. These con- 

 stitute the only modes in which any one organism can 



