A. E. HILTON ON THE NATURE OF LIVING ORGANISMS. 45 



doubt that liquefactions and gelations of the plasm, however 

 caused, play a leading part in the manifestations of life. Alter- 

 nations of heat and cold, light and darkness, moisture and dryness 

 affect the organism in different ways ; and without ready means 

 of readjustment it would suffer, if not perish. The necessary 

 adaptation to environment is accomplished by the vital machinery 

 working automatically in either direction. An oscillating equili- 

 brium of reversible or compensatory processes is maintained, or 

 the life-phenomena fail. 



This automatic reversibility of the living mechanism has another 

 important result. It tends to reproduction. It not only saves 

 the individual from premature decease ; it also makes for the 

 preservation of the race. Processes of encystment, which are 

 rejuvenescent as well as protective, are commonly reversed 

 processes of development. Reproduction is brought about by 

 some portion of the mature organism reverting to a close approxi- 

 mation of the embryonic substance from which it grew. First 

 structures are simple, rather than complex ; and the wonderful 

 fertility of lower organisms is due to the ease with which their 

 plasm reverts to its embryonic state. When reproduction is 

 sexual, it implies that, owing to higher specialisation, the primitive 

 condition cannot be regained except by fusion of sperm- and germ- 

 cells. When that primitive condition is reached, the backward 

 process is arrested, and the ensuing reversal of the machinery 

 causes the development of the offspring. 



Probably the most distinctive property of plasm in general is 

 its strongly catalytic character. That, of course, does not entirely 

 distinguish it from lifeless matter, because catalysis is a familar 

 fact, even in inorganic chemistry. It is well known that certain 

 compounds, a and b, which affect each other when in contact, 

 react much more vigorously if, at the same time, a third substance, 

 C, is present ; while c, although probably participating in the 

 intermediary stages of the reactions, recovers its identity, and 

 at the end of the process is unaltered. In such cases, c is the 

 catalyser ; and it is the possession of this property in a pre-eminent 

 degree which gives plasm its remarkable power of assimilating 

 or modifying the food materials which form its working stock. 



