22 JOURNAL OF THE [January, 



and red rays are inactive or neutral. Like the movement of 

 swarmspores, as well as that of desmids, the locomotion of 

 diatoms is influenced by light. If water containing these organ- 

 isms is shaken up and poured out into a shallow vessel and set in 

 a bright place, the diatoms will quickly separate themselves from 

 the dirt and debris with which they have become mixed, and,, 

 rising to the surface, will gather together on the side of the vessel 

 most under the influence of the light. The philosophy of this 

 operation appears to be that the light promotes the chemical 

 action, whatever it may be, within the protoplasmic contents of 

 the frustule, which develops a force that reacts upon the water 

 and results in the propulsion of the diatom in the direction of 

 the light. I cannot myself understand why such a result should be 

 produced if the cause of locomotion were the direct action of pseu- 

 dopodia or cilia, unless we are to admit a certain degree of pur- 

 poseful control of those organs towards the definite end of seeking 

 the light, — which calls for an exercise of choice on the part of the 

 diatom, at least equal to the apparent volition involved in the 

 food-seeking movements of unquestioned animalcules. On some 

 such theory as that of chemical or osmotic action promoted by , 

 light, however, I can easily imagine a reasonable explanation of 

 the phenomenon not inconsistent with the generally accepted 

 belief in the diatom's essentially vegetable nature. 



By adopting this hypothesis, as I understand so high an 

 authority as Prof. H. L. Smith has done, I confess that it 

 seems to me we best get rid of that bafiling question as to voli- 

 tion; for if the diatom's movements are merely a species of helio- 

 tropism, there is no more room for will or mind in the matter 

 than there is in the case of any of the shrubs which turn their 

 flowers or leaves in a given direction to the sun. Surely the 

 evidence of choice in the action of the diatom is no greater than 

 it is in the wonderful discrimination exercised by the roots, 

 tendrils, and tentacles of higher plants, as shown by the delicate 

 experiments of Charles Darwin and his son. I am aware, how- 

 ever, that it is laid down, as a fundamental axiom of psychology, 

 that the function of selective discrimination is the root-principle 

 of mind. But this train of thought leads to deeper questions 

 than it is proper for me to enter upon at this time. I have 

 endeavored, thus far, to confine myself mainly to those phases 



