UNICELLULAR FORMS 53 



BEHAVIOR, that is, of hormones, of nerves, and of responses to the 

 environment. 



(5) Principles of structural and of functional unity; the science 

 of ANATOMY and the relations between the anatomy and physiology 

 of the organism. 



(6) Principles of progressive complexity of structure and of 

 function in relation to the history of animal life on the earth and 

 in relation to the orderly classification of present-day animals; the 

 sciences of evolution, and taxonomy or classification. 



(7) Principles relating to perpetuation of kind; the sciences of 

 reproduction, of development of the adult, that is, embryology, 

 and of inheritance. 



(8) Principles of adjustment to contact with the environment; 

 the sciences of ecology, that is, distribution of animals, and of 

 adaptation. 



(9) Principles of association with other organisms; parasitism, 

 animal communities and animal sociology. 



(10) Principles of changes with time; the sciences of growth 

 and of SENESCENCE or aging. 



In reality none of these can be discussed independently of the 

 others and each includes materials that comprise one or more ex- 

 tensive biological science. Hence it is clearly impossible to treat 

 each or any exhaustively here. Nor is it desirable, for the major 

 objective we have set is a comprehension of the fundamental rela- 

 tions between these groups of principles, in the lives of animals. 

 Hence we are limited to illustrative examples from all these groups 

 in order to demonstrate just what an animal is and what it does. 

 A necessary preliminary to the understanding of the operation of 

 these principles in the case of any animal is a knowledge of the 

 structure of the animal and obviously the logical organism with 

 which to begin their study is the organism that is most simple. 



The protozoa are whole organisms that consist of single cells. 

 Here one finds the least complicated application of the principles 



