220 JACQUES LOEB AND HARDOLPH WASTENEYS 



nia, appears brightest to the human inflicted with total color 

 blindness, he thinks he has proved that Daphnia is also totally 

 color blind. 



The number of objections to this kind of reasoning is 

 considerable." 



Nobody has yet proved that the heliotropic reactions of ani- 

 mals are determined or even accompanied by any sensations of 

 brightness and it is difficult to see how such a proof can ever be 

 furnished. It is plainly unwarranted to assume that every 

 motion of animals induced by light is accompanied by or is the 

 expression of sensations of brightness or of color. The excised 

 iris of the shark (and of other animals) contracts under the in- 

 fluence of illumination and Magnus has shown that the yellowish- 

 green part of the spectrum is most efficient in this case. It 

 would be arbitrary, to say the least, to state that the excised iris 

 has sensations of brightness and that these sensations make it 

 contract; and yet it is difficult to see why such an assumption 

 should be more arbitrary than a similar assumption in the case 

 of the flagellate Chlamydomonas or the heliotropic larvae of 

 Balanus. One wonders also whether we are supposed to assume 

 that Hammond's heliotropic machines are guided by sensations 

 of brightness or of color. 



The assumption of Hess might be given some consideration if it 

 could be shown that totally color blind human beings are posi- 

 tively heliotropic, i. e., are irresistibly drawn to the source of 

 light; but nobody has ever heard of such a case. The human 

 being is the only one about whose sensations we have definite 

 knowledge and as long as we are unable to prove for the human a 

 connection between positive heliotropism and the sensations of 

 brightness we have no right to take such a connection for granted 

 in the lower animals. 



One wonders also what interpretation is to be put on other 

 tropisms, such as galvanotropism or geotropism, if we accept the 

 validity of Hess's viewpoint, since it is only logical to treat all the 



^ An excellent criticism of Hess's ideas and experiments has been given by 

 W. F. Ewald, Arch. f. Entwcklngsmech., 1915, 37, 581. We are using some of his 

 arguments in this paper. 



