REGULATION OF REACTIONS 267 



in lobster larvae, and by various investigators in a number 

 of other organisms. 



It will thus be seen that reversal in the sense of orienta- 

 tion caused by the effect of light is widely distributed among 

 living organisms. Is this effect of light due to stimulation 

 caused by the process of changing the intensity, as in case 

 of orientation in Euglena, for example, or the contraction 

 of Hydroides ? Or is it due to continuous illumination, as 

 in case of the activity of many organisms ? In other words, 

 is it due to the time rate of change, or to constant intensity ? 

 I have frequently observed that Chlamydomonas, Euglena, 

 Volvox, and other similar forms do not become negative at 

 once if the light intensity is suddenly increased above the 

 optimum. These organisms must be exposed to the higher 

 intensity for some little time before the sense of reaction is 



of twenty minutes "; and on the following page he says, "I again took some 

 animals which had become positively hehotropic in the north room, and 

 convinced myself first of all that at a constant temperature of 20° C. they 

 would become negatively heliotropic in direct sunlight in a few minutes. 

 I then returned them to the north room, and here the animals again be- 

 came positively heliotropic at the same temperature in the course of fifteen 

 minutes." 



It would be difficult to state in more exphcit terms that the nauplii of 

 Balanus and the larvae of Polygordius are negative in strong fight and 

 positive in weak than Loeb has done in the passages quoted from his pub- 

 lication of 1905, and it would be equally difficult to state more explicitly 

 that they are not negative in strong and positive in weak light than he has 

 stated in his address of 1909. 



If these organisms are negative in strong and positive in weak light, as 

 Loeb's experiments indicate, it is evident that their reactions tend to keep 

 them in light of moderate intensity, an idea quite in harmony with those 

 Loeb rejects in his recent address. As a matter of fact it is not at all diflicult 

 to find Chlamydomonas, Euglena, Volvox, or any other similar organisms 

 in such a state that they are neutral in a given light intensity, positive in a 

 lower intensity, and negative in a higher. I have repeatedly observed this in 

 all of these forms as well as in several others; and in case of Volvox I have 

 many times observed, as stated elsewhere, that the colonies collect in great 

 numbers in the open spaces between pond-lily leaves and other water plants 

 on dark, cloudy days, but that they collect in shaded places when the sun 

 is bright. 



