THE EVOLUTION OF ANIMAL FUNCTION 323 



made in the actual physiological properties of the cells within 

 the organism, as when a nerve-cell has acquired the capability 

 of more rapid conduction, or a gland-cell has become peculiarly 

 sensitive to the stimulus afforded by the products of activity 

 of certain other cells within the organism. The latter changes 

 are to be recognised as distinct from the former because they 

 constitute change of the actual inherent properties of the cells. 

 This distinction is not invalidated by the obvious fact that, in 

 the complex of organic reactions, a change of functional capability 

 will seldom occur, without in some way involving also a change 

 of opportunity for cells other than those directly concerned. 



If we accept provisionally this grouping of the evolutionary 

 changes which organisms have undergone, we see at once that, 

 for the investigation of the first category of changes, those of 

 functional opportunity, there are two methods of observation 

 available, the morphological and the physiological. It needs 

 no showing that the functional opportunities of cells may be 

 altered either by morphological change, as in the example given 

 above of the formation of a digestive cavity, or by the physio- 

 logical change of other cells, as for example if one group of cells 

 should secrete a chemical substance which should act as a 

 stimulus to others contained in the organism. For the study 

 of changes in functional capability the direct observation of 

 function is the only possible method, and inference from 

 morphological observation can have no voice whatever in the 

 matter. The knowledge that certain cells forming part of an 

 organism have been set apart for digestive purposes by the 

 morphological formation of an alimentary canal, may tell us 

 what functions those cells have to perform, but can tell us 

 nothing whatever about their properties. The cells may have 

 retained unchanged all the properties possessed by their 

 predecessors in phylogeny at the time when no special cavity 

 was set apart for digestion. They may, on the other hand, have 

 been profoundly modified for the special duty to which the 

 morphological change has limited them. 



Here then we find two categories of evolutionary change 

 open to inquiry — the one presenting problems both to mor- 

 phology and to comparative physiology, the other capable of 

 investigation by the methods of physiology only. The question 

 I wish to ask is whether the contribution which the com- 

 parative study of function might make towards the solution 



