A QUARTER-CENTURY OF PLANT PHYSIOLOGY 6 



'80's with those of the last decade, hes in the gradual develop- 

 ment of a more and more thorough-going determinism. The 

 echoes of an age-old struggle between mysticism, vitalism, and 

 so forth, on the one hand, and mechanism or determinism, on 

 the other, are still rolling on our scientific horizon, but the orderly 

 quiet of physical science now extends into almost every phase of 

 physiological thought. As Jacques Loeb has aptly remarked, 

 no matter what may be the emotional attitude of a physiological 

 investigator in these later days, such a one is sure to adopt a 

 deterministic philosophy as soon as he steps into his laboratory 

 and begins an experiment. What we may think or feel about 

 questions that are too complex to be subjected as yet, to experi- 

 ment, is of little consequence in modern science, but rational 

 experimentation is practically impossible without the con\'iction 

 that phenomena are rigidly determined by pre-existing con- 

 ditions. \Miile there may come other periods, as in times of 

 great wars, when the stress of life itself may compel the tem- 

 porary putting aside of reason, in favor of more elemental 

 emotions, yet it may be safely predicted that physiological 

 advancement will always be guided primarily by the mechanistic 

 or detenninistic conceptions that have come to us from the 

 non-biological sciences. Progress has always been in this 

 direction; as Cowles has said, many \dtalistic interpretations 

 have been replaced by mechanistic ones, but the reverse change 

 has not yet occurred. 



Tentative nature of physiological conclusions. In the twenty- 

 five years here considered, physiology, along with other natural 

 sciences, has undergone a marked alteration in respect to the 

 final value that it places upon its interpretations. It is an in- 

 teresting fact that increased knowledge of plant and animal 

 processes has thus far tended rather to emphasize their extreme 

 complexity than to establish their conditional relations in any 

 final way. During the last quarter-century we seem, indeed,^ 

 to have been mainly engaged in discovering new problems, rather 

 than in solving the problems dealt with by earher workers. 

 In many cases the earher problem has been analyzed into several 

 component problems, each one of the latter appearing as difficult 



