LIVING MATTER 29 



place to place. Further, if we set up a current in 

 the water in which the amoeba is floating we shall 

 find that it always moves against that current. 

 This is a function of most unicellular forms and of 

 more highly organised creatures as well, for water- 

 fleas and other small Crustacea will swim away 

 from the fluid which is rushing into the pipette 

 designed to capture them. It is clear that this 

 disposition to swim against the stream must be of 

 considerable advantage to the amoeba in keeping 

 it in its place in spite of the natural and ordinary 

 currents in the water in which it may be living, or 

 those other adventitious currents which may be set 

 up from time to time by the passage of other 

 creatures or by other means. 



This power of being able to move itself is one 

 of the most prominent and striking differences be- 

 tween the living creature and the non-living sub- 

 stance, and it was seized upon by St. Thomas 

 Aquinas for his definition of a living thing. Thus 

 he defines it : " Ilia proprie sunt viventia quae 

 seipsa secundum aliquam speciem motus movent " ; l 

 and again, " Ens vivum est substantia cui convenit 

 secundum suam naturam movere seipsam ". 2 But 

 it must not be supposed that in making this defini- 

 tion, which is a natural one and the most accurate 



l Summa TheoL, 1, 9, 1, art. 1 c. 

 Ubid., la, 9, 18, art. 2 c. 



