LIGHT AND MOVEMENT 29 



machines the activities of which were expHcable by the same physical 

 laws.i As a young colleague of the botanist, Sachs, at Wiirzburg, he 

 appreciated the immense strides his friend had made in the interpreta- 

 tion of the responses of plants and unicellular organisms to light, and 

 applied the same techniques to the animal world. All voluntary and 

 instinctive reactions of animals he considered to be determined by 

 internal and external forces, the majority of their responses thereto 

 depending upon their bilaterally symmetrical structure. Thus, in the 

 simple reaction of an animal going towards or away from a light, if the 

 velocity of the chemical reactions in one eye is increased, the equality 

 of " tonus " in symmetrical muscles on the two sides of the body is 

 altered so that the animal is compelled to change its direction of 

 locomotion ; as soon as the plane of symmetry becomes directed 

 through the source of light, muscular tone becomes equalized and the 

 animal progresses straight ahead until some other asymmetrical 

 disturbance changes its direction of motion. Any other form of 

 energy, he claimed, acted in the same way as light, so that the animal, 

 which may appear superficially to move purposively and of its own 

 will, is in reality forced to go where it is carried by its legs or wings. 

 Animal conduct was thus interpreted as consisting of forced move- 

 ments, a conception very different indeed from the anthropomorphic 

 and teleological views prevailing throughout the nineteenth century. 



Loeb pursued his theories with immense activity and application, 

 and defended them with unusual vigour and stubbornness. It soon 

 became obvious, however, despite his warm advocacy, that the 

 intricacies of animal behaviour could not be contained within a theory 

 so simple. Moreover, its all-embracing character and its rigidity 

 readily opened it to attack as observations on the complexity of the 

 conduct of animals multiplied. Jennings (1904-6) first showed that 

 the reactions even of Protozoa could not be explained in this decep- 

 tively simple way, and the automaticity of the reactions of animals was 

 challenged and disproved by many workers, ^ but by none more 

 conclusively and consistently than by samuel o. mast (1871-1947) 

 who proved to be Loeb's most violent and successful opponent (Fig. 12). 

 Undoubtedly Loeb had swung the pendulum too far. A considerable 

 reconciliation between the two opposing views was put forward by 

 Kiihn (1919), but general accord has by no means yet been reached. 



It is probably true that the mechanical evidences of organic 

 activities ultimately conform to the rules of chemistry and physics ; 

 but these rules have yet to be formulated ; nor — most fortunately — is it 

 necessary to await a complete explanation in fundamental terms before 



1 See especially his Mechanistic Conception of Life (1912). 



2 V. Buddenbrock (1915), Bierens de Haan (1921), Alverdes (1932), Russell (1938), 

 and others. 



