64 VITALISM AND SCHOLASTICISM 



creature and it is also able to transport itself 

 from 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 disposi- 

 tion to swim against the stream must be of 

 considerable advantage to the amoeba in keep- 

 ing 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 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 differ- 



Thc living ences between the living creature and the non- 

 object 

 moves living substance, 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 mo vent";* and again, "Ens vivum 

 est substantia cui convenit secundum suam 



* Summa Theol., la, 9, 1, art. 1 c. 



