General Physiology of Animals. 335 



only morphologically, but physiologically as well ; that, in 

 short, the key to the understanding of the structure and life- 

 history of animals, as of plants, is morphological and phy- 

 siological differentiation of a primitively simple or generalised 

 type. 



SECTION III. GENERAL PHYSIOLOGY OF ANIMALS. 



In our study of plants we found it advisable to devote 

 a section to a general survey of the subject of physiology, 

 or the mode in which the various organs of the plant per- 

 formed their functions, and the part each played in the 

 general phenomena of life. In the preceding survey of the 

 chief points in the organisation of the representatives of 

 the Metazoa which we have chosen as typical, physiology 

 .has been incidentally dealt with in connection with mor- 

 phology. A section may, however, with profit be devoted 

 to a general summary of the principles of animal physi- 

 ology in particular, and the comparison of animal with plant 

 physiology. 



We may begin our review by noting that the funda- 

 mental principles of plant and animal physiology are the 

 same. In both we have an organism essentially composed 

 of protoplasm, subdivided into cells which are again modi- 

 fied in various ways according to the special function or 

 functions they have to perform. Protoplasm, in short, as 

 already stated (p. 33), is a highly complex store of potential 

 energy, in virtue of the possession of which the plant or 

 animal is able to perform certain duties related to the main- 

 tenance, partly of tribal, partly of individual life. Since 

 the phenomena which characterise animal life are manifested 

 only as a result of the decomposition of some of these pro- 

 toplasmic compounds, so that a certain amount of potential 

 energy becomes kinetic, it follows that, as in the case of the 

 plant, the physiology of an animal has two aspects: first, the 

 anabolic or constructive aspect; and secondly, the katabolic 



