54 ESSENTIALS OF BIOLOGY 



analysis of the functioning of an organ reduces to the processes 

 that take place in the bodies and nuclei of the characteristic cells. 

 We shall see, of course, that individual cell activities in an organ 

 are integrated and that even the activities of organs and organ- 

 systems are integrated. Nevertheless the activities of a muscle 

 reduce to the activities that proceed in the individual muscle- 

 fibres, or cells, and the secretion of saliva by the sub-maxillary 

 gland reduces to activities that are, in the main, those that occur 

 in the cells that form the gland acini. 



15^. Structure and Phylogeny. The matter of the preced- 

 ing sections deals only with the main kinds of animal structure, 

 apart from any hypotheses of the ways in which those types of 

 structure have evolved. Also it suggests what are the animal 

 bodily parts that we shall regard as functioning in the following 

 sections. 



But much interest in animal morphology centres round 

 classifications. The ways in which systematists arrange animals 

 into species, genera, families, classes and phyla depend upon 

 structure. Phylogenies show in what ways all the races, species, 

 genera, etc., are related and how one race, species, etc., has evolved 

 from some other one : They are " family trees." Structure, 

 whether studied in the embryos and adult forms of recent animals, 

 or in the fossils of extinct ones, appears to be the only way in which 

 it may be possible to trace out evolutionary histories. 



