June 23, 1911] 



SCIENCE 



965 



simple reflex responses. The question re- 

 mains open whether the stimulus is produced 

 by the direct action of light on the sensitive 

 surface of the animal or by changes in inten- 

 sity of the stimulus caused by the lateral 

 movements of the body. The question in this 

 as in many other cases is diiBcult to decide. 

 Analogy with the method of orientation in 

 Euglena leads the author to incline toward the 

 latter alternative, but the experimental data 

 obtained did not enable him to solve the prob- 

 lem. In either case orientation in this form 

 is apparently as automatically regulated an 

 activity as one might expect according to the 

 well-known theory of Loeb. 



The problem of whether the rays of light 

 per se or variations of light intensity per se 

 afFord the stimuli which cause orientation is 

 discussed in connection with accounts of ex- 

 periments on the blow fly, earth-worms and 

 other forms. There is much to be said for the 

 efiiciency of both these factors in special cases. 

 There are instances in which the shock of 

 transition from one light intensity to another 

 causes a marked reaction. Light in other 

 cases has a constant stimulating effect apart 

 from any shock of transition. In an organism 

 going towards or away from the light devia- 

 tions from the direct course produce a change 

 of intensity of light falling on its two sides; 

 that the animal turns directly and automat- 

 ically into line does not of itself enable one to 

 decide whether it turns because it is more 

 strongly stimulated on the one side than on 

 the other, or whether it turns on account of a 

 change of light intensity. There have been 

 few attacks upon this particular problem and 

 the author does well to bring it into more 

 prominence than it has usually been accorded. 



Part III., entitled General Considerations 

 on Eeactions to Light, deals chiefly with the 

 various methods of forming groups in regions 

 of certain optimum illumination, reactions 

 which involve no directive effect of light, and 

 the factors which induce changes in the pho- 

 totactic response. There is no discussion of 

 any theoretical attempt to explain the reversal 

 of phototaxis and the author advances no 

 views of his own. 



Part IV. deals with reactions to light of dif- 

 ferent wave-lengths. Much work in this field 

 has given inconclusive results on account of 

 failures to obtain pure monochromatic light 

 and on account of not taking into considera- 

 tion differences in energy of different wave- 

 lengths. While in general the rays toward the 

 violet end of the spectrum are more potent in 

 producing the heliotropic response in both 

 plants and animals, the rule is not without 

 exceptions in both kingdoms. As the writer 

 states, this is probably due to the fact that 

 some chemical processes are accelerated by 

 light of certain wave-length, while other chem- 

 ical processes are accelerated most by light of 

 a different wave-length. Sometimes mono- 

 chromatic light is more potent than white 

 light, a fact which suggests that certain rays 

 tend to inhibit or reverse chemical reactions 

 which are caused by rays of a different wave- 

 length. Mast's treatment of the effect of 

 light of different wave-lengths can not fail to 

 be very helpful to all workers in this rather 

 difficult subject. 



The final chapter on Theoretical Considera- 

 tions contains, besides a general summary of 

 results, a renewed attack upon Loeb, a short 

 consideration of the much-refuted vitalism of 

 Driesch, and an exposition of the general posi- 

 tion of Jennings to which the author is a very 

 close adherent. After stating that according 

 to Jennings's views the reactions of an organ- 

 ism are fundamentally purposeful and that 

 they depend upon previous activity as well as 

 present external conditions, the author ex- 

 pounds the position of Jennings as follows: 

 " Eeactions are defined as changes in the ac- 

 tivity of organisms. Such changes may occur 

 under constant external conditions. They are 

 therefore due primarily to internal changes. 

 External factors cause reactions not directly, 

 but indirectly, by altering internal processes 

 (physiological states). Variability in reac- 

 tion to given external conditions is due to 

 changes in physiological states. If an organ- 

 ism responds to light of a given intensity in a 

 given way now, and to the same intensity in 

 another way later, it is because the physiolog- 

 ical state of the organism has changed." Pol- 



