ANIMAL BIOLOGY. [Chap. I. 



is decomposed, and oxygen set free into the air. In the animal 

 the carbonic acid gas is recomposed and breathed forth into the 

 air. 



The higher animals thus differ from the higher plants (1) in 

 that they recompose the carbonic acid gas which the plants 

 decompose ; (2) in the nature and amount of their activities ; 

 and especially (3) in requiring protoplasmic food-stuff, which 

 must undergo a process of digestion before it can be assimilated. 



Having now gained some idea of the nature of those living 

 things which it will be our task to study, let us proceed to 

 consider what is the nature of the problems that are likely to be 

 suggested by the study of animal organisation and animal life. 

 And that we may not fall into generalities beyond the reach of 

 the student,, let us consider the special case of such an animal as 

 the common frog. In the first place, the fully-grown frog has 

 a tolerably constant external form and appearance, by which 

 it may always be recognised and distinguished from other 

 organisms, such as the toad, or the edible frog of the Continent. 

 Dissection after death further shows that it has a definite and 

 tolerably constant internal structure. A number of organs such 

 as heart, liver, stomach, kidneys, are found within the body, 

 and these have a constant form and constant relations to the 

 nerves and blood-vessels which ramify throughout the body. 

 And minute examination with the microscope further shows 

 that there is a definite and constant minute structure. The 

 organs are seen to be built up of cells or cell-products aggre- 

 gated in special ways into what are called tissues. So that 

 we have to consider : (1) the general form and structure of the 

 organism ; (2) the special form and structure of the organs ; and 

 (3) the minute form and structure of the tissues. 

 . But the frog does not stand alone among organisms. Hence 

 it is necessary to compare its general structure, the structure of 

 its organs, and the structure of its tissues, with the general, 

 organic, and minute structure of other organisms, with the 

 object of ascertaining what are the points of resemblance and 

 the points of difference. By this means the range and import- 

 ance of the structural problems is enormously increased. 



