IRRITABILITY 185 



the solution throughout the cell, insure the thoroughness 

 with which the protoplasm will be affected. The complexity 

 of protoplasm in composition and structure, and the physi- 

 cal and chemical properties of the cell-sap which is every- 

 where within it, help us to see that protoplasm is neces- 

 sarily as well as actually the most unstable and the most 

 complex, structure known. 



Comprehending these facts, we see reasons for the sensi- 

 tiveness of protoplasm to outside influences. But lifeless 

 protoplasm — imagining such a thing for the moment — al- 

 though it possesses all this complexity of structure and of 

 composition, and is therefore sensitive to influences from 

 without, is not the seat of the physiological processes, of 

 the destructive and constructive chemical changes, which are 

 constantly going on in living protoplasm. Indeed the dry 

 seed is far less sensitive, as we have already seen (pp. 9, 10), 

 than the same seed after water has been absorbed and ger- 

 mination has begun. Wherever there is actively living proto- 

 plasm, i.e. a complex structure among the compounds of 

 which and within which chemical changes are constantly 

 taking place, we have the conditions for irritability : the 

 more complex the structure and its component and enclosed 

 compounds, or the more varied and the more rapid the 

 chemical changes taking place in the structure, the greater 

 will be its sensitiveness to external influences. The higher 

 the organism, the more complex is its structure, the more 

 varied or the more rapid are the chemical changes taking 

 place in it, and therefore it is the more sensitive. The or- 

 ganism is dormant, unsensitive, unirritable, when its physi- 

 ological processes are slow or simple ; the organism is dead 

 when its physiological chemical changes cease and its struc- 

 ture breaks down. Its component molecules may still be 

 there intact after the organism ceases to live, but their ar- 

 rangement is changed, the sensitiveness of the whole struc- 

 ture and of all its parts is diminished in proportion as the 

 arrangement of the molecules is modifled. 



The irritability, then, of living organisms consists in a 

 sensitiveness to external conditions which is due to the 

 complexity in structure and in composition of the organ- 



