THE EVOLUTION OF ANIMAL FUNCTION 479 



the general use or application of an organ and the functional 

 events which go on in the individual cells of which it is 

 composed. 



But even when a function has been referred to the individual 

 cell by which it is performed, analysis has not yet gone deep 

 enough. There is still a possibility of confusion between two 

 quite independent factors which determine the functional 

 behaviour of any cell. The actual functional behaviour of a cell 

 is not settled entirely by the properties of the cell itself. It is 

 the product of the capability of the cell to respond to a given 

 external cause by which it is disturbed, and of the particular 

 disturbing cause which is acting at the time. This distinction 

 between functional behaviour and capability may sound trite 

 and obvious ; but it is too easily slurred over in the treatment 

 of the evolution of function. We must therefore digress at this 

 point in order to define the matter more clearly. 



The cells which constitute an organism react one with another 

 in many different ways. Their mutual reaction constitutes the 

 whole system of co-ordination or " integration " under whose 

 working a conglomeration of unlike cells takes on the character 

 of a regulated organism. A nerve cell may act upon another 

 nerve cell or upon a muscle cell or gland cell by the direct means 

 of the nervous impulse. One cell of the digestive mechanism 

 may pour into the blood-stream a chemical secretion which is 

 carried to and promotes the activity of other digestive cells. 

 Fatigued muscle cells may deliver to the blood a product which 

 passes to the brain, there- to throw into activity certain nerve 

 cells, and from these nerve cells impulses may pass down to 

 control the muscle cells of respiration. All these are examples 

 of the co-ordination and direction of the functional behaviour 

 of one cell by the activity of another. And it is clear that this 

 co-ordination involves the entire activity and structure of the 

 organism. It depends, of course, upon the sequence and time 

 relations of the individual activities of the cells concerned. But 

 it involves also the anatomical relations of organs, their blood 

 supply, and the paths of their nervous connexion within and 

 without the central nervous system. Evidently, then, a difference 

 of behaviour between two cells, in so far as it may spring partly 

 from the reaction with other cells, cannot be thought of as a 

 pure difference of function. It may be essentially a difference 

 of structure, a rearrangement of nervous connection, for example. 



