THE TROPISM THEORY OF JACQUES LOEB 359 



we get a tropism. This definition of an animal, however, is false, 

 and, in consequence, so are the conclusions that are drawn from it. 



The animal is rather a mechanism of purposeful adaptive 

 construction for the tasks of self -maintenance and reproduction. 

 This is an unassailable truth; therefore it follows that the sepa- 

 rate organs, for example, the eye, have a definite purpose, as 

 the adaptive structure of the eye well shows, and even though 

 this eye causes nothing but a heliotropic movement, in the last 

 analysis, the reflex arc which enables the movement to take place 

 must be considered as a purposefully constructed mechanism. 

 Therefore this very mechanism is the cause of the tropism and 

 the tropism theory is entirely superfluous. This argument re- 

 futes the tropism theory so far as those lower animals are con- 

 cerned whose eyes are only heliotropic in function. 



One exception must be made. In highly developed sense 

 organs the matter is somewhat more complicated. If, for ex- 

 ample, an eye is adapted for perceiving images as well as for 

 allowing heliotropic movements, one might assume that here 

 image perception alone is the purpose of the eye, while heliotropism 

 is but one of nature's unintended by-products. 



In this case, one might almost assent to Loeb's view, were it 

 not for those arguments already known to us which are based 

 upon the details of the movement, and which entirely refute 

 the theory. The existence of a reflex arc makes the theory 

 superfluous, at least in many cases, and robs it of any content. 

 Tropism then finds its cause in the reflex arc itself. It can 

 neither be " explained " away nor denied, it is a simple fact of 

 observation which leaves no room for a theory. 



In all further cases, where such a reflex arc is not clearly 

 demonstrable, but a nervous system exists which connects mus- 

 cles and sensory epithelium, it is most probable that only our 

 lack of operative skill prevents us from finding the reflex arc, 

 and that it is- perhaps never absent. This again lessens the 

 theory's right to exist. I should like to bring such a case under 

 closer consideration because it demonstrates so clearly the 

 entirely unproved assumptions upon which the supporters of 

 the tropism theory depend. Davenport (1897) writes in regard 

 to the negative heliotropism of the earthworm: 'The sun's 

 rays may fall horizontal and at right angles to its axis. Then 

 the rays strike it (the earthworm) laterally or in other words, it 



