INTRODUCTION 17 



extended surface for gaseous exchange. The glands are 

 organs in which are concealed great surfaces through 

 which products of cell activity find an outlet. 



The specialization which groups the cells of the animal 

 body in a number of classes each with its definite work to 

 do also entails the dependence of each class upon the 

 others. If we compare the life of a savage with that of a 

 civilized man we shall find an analogy not too far fetched 

 to be helpful. We have seen that the free-living cell is 

 self-sufficient, and, indeed, its chances of survival are bet- 

 ter when there are but few of its own kind in the neighbor- 

 hood. Such organisms are competitive rather than co- 

 operative. Somewhat in the same way the solitary savage 

 may be capable of self-maintenance, having the skill to 

 find and prepare his food and after a fashion to shelter 

 and clothe himself. The civilized man is accustomed to 

 look to many other men and women to supply his needs. 

 Yet if the man, like the cell, loses something of ruggedness 

 and resourcefulness through becoming a member of a com- 

 plex society, he evidently gains time and opportunity to 

 concentrate his efforts upon a special pursuit. It is very 

 much the same with the cell. Our analogy fails, as such 

 devices are prone to do, when we consider how the civil- 

 ized man may become a hermit or a Robinson Crusoe, 

 whereas no single cell detached from one of the higher 

 forms can exist by itself for any length of time. 



Co-ordination. We have emphasized above the services 

 rendered to the organism by its internal medium. The 

 composition of the circulating fluid is influenced by the 

 ever-varying activities of all the organs and tissues. 

 Accordingly, a contribution made to this medium by 

 any group of cells may conceivably modify the conduct 

 of any other group. We shall meet with numerous 

 instances of such influence exerted through the chemical 

 products of one organ upon another or upon the system as 

 a whole. When we speak of an animal as an individual 

 we imply that the parts of the body constantly interact. 

 It is this interaction which makes it appropriate to regard 

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