io SENESCENCE AND REJUVENESCENCE 



vital principle, because our knowledge of the organism is insufficient. 

 Only when we have exhausted physico-chemical possibilities and 

 found them to be inadequate shall we be justified in searching else- 

 where for the basis of life. 



But there is one point of particular interest in connection with 

 the neo-vitalistic hypotheses. They are a logical consequence of 

 the corpuscular theories of heredity and organic constitution and 

 development, such as the theory of Weismann. These theories 

 were widely current at the time when the neo-vitalistic school 

 arose. They themselves are fundamentally "vitalistic" in char- 

 acter, whatever their assertions to the contrary. An orderly pro- 

 gressive development of a definite character is inconceivable in an 

 organism composed of a very large number of independent ultimate 

 units each capable of growth and reproduction, except under the 

 influence of some controlling and directing principle distinct from 

 the ultimate units themselves. If such theories represent the last 

 word of science concerning the physico-chemical constitution of 

 the organism, then we must all be vitalists, whether we admit it 

 or not. But if the controlling and determining principle, entelechy 

 or whatever we choose to call it, is indispensable, why must we 

 complicate matters by assuming the existence of a multitude of 

 discrete ultimate units of one kind or another ? Why not give the 

 entelechy a task worthy of it and assume that all parts of the organ- 

 ism are essentially alike and equipotential ? This is practically 

 what Driesch has done. The entelechy determines localization 

 and development and uses physico-chemical processes to effect its 

 ends. 



The trend of biological thought has undergone change during 

 the past twenty years. The development of experimental methods 

 on the one hand and the development of the physical sciences on 

 the other have contributed to alter our conception of the organism 

 and today there is less basis for vitalistic theory than ever before. 

 Even the theory of Weismann and other morphological theories of 

 the organism are giving place to theories of a different type, and 

 while many other attempts will undoubtedly be made in future to 

 demonstrate the indispensability of some sort of vital principle, 

 the analysis and synthesis of science, proceeding step by step, test- 



