THE TROPISM THEORY OF JACQUES LOEB 355 



neutral ground in regard to this interesting question. We can- 

 not prove that the phototropism of the Mysidae is evidence 

 against the theory, but neither can Loeb show that it is evidence 

 in his favor. 



THE IMPOSSIBILITY OF FINDING THE AXIS OF TURNING 



The three last-mentioned arguments against the tropism 

 theory have been unaccountably overlooked by most critics 

 until the present time. The same is true of a fourth which will 

 be considered next. 



Any bilaterally symmetrical organism that is sensitive to light 

 will serve as an example, but to select one of Loeb's own, let us 

 take the winged aphid. If this insect is illuminated from the left 

 side only, (meaning that the source of light is neither above nor 

 below, but exactly on a level with it), it will turn immediately 

 and fly in a straight line to the source of light. This is a very 

 clear case of heliotropism, in Loeb's sense. Illumination of one 

 side, result: unequal movement of the two sides until a position 

 is reached which is symmetrical with respect to the source of 

 light; finally, flight in a straight line to the light. 



Now how much of this can the tropism theory explain? Evi- 

 dently, setting aside some considerations to be discussed later, 

 only the point that the two sides of the body move differently. 

 Now from this, logically, a turning of the whole organism 

 around some axis lying in the plane of symmetry, xy, must 

 result, which turning continues until the plane of symmetry 

 coincides with the plane xyL (L = point of light) . But that 

 is all that can be obtained from the tropism theory! The 

 theory does not reveal around which one of the innumerable 

 axes of this plane a turning takes place. And yet, in order to 

 reach the source of energy, the turning must be around a definite 

 axis, namely, one perpendicular to the line joining the organism 

 with the source of energy. Any theory which deals earnestly 

 with the present problem should explain how the turning takes 

 place around just this one axis, or in other words, how the move- 

 ment can occur in a few definite flight-muscles out of a great 

 number. Were Loeb's theory our only resort it would be neces- 

 sary to assume a miracle to understand how the aphid ever gets 

 to the light. If point A is the animal, point L the light, and if 

 I assume that the turning required by the theory is ended, and 



