THEORETIC CONSIDERATIONS 367 



crease as well as to increase of temperature, even if the 

 change is only slight. 



(5) A given condition of illumination may inhibit one 

 kind of movement in an organism and cause movement 

 of another kind. When the oral end of Hydra viridis 

 is fully illuminated the swinging about the point of at- 

 tachment is inhibited and locomotion is produced. (See 

 Chapter VIII). 



(6) An increase in the general illumination of an organ- 

 ism may cause an increase in activity, while a sudden de- 

 crease of intensity causes a still greater increase in activity 

 in the same organism at the same time. If the light inten- 

 sity on a Volvox colony under certain conditions is increased, 

 all of the zooids in the colony become more active, but those 

 on the shaded side of the colony become most active. The 

 rotation of the colony on the longitudinal axis causes a sud- 

 den decrease of intensity on the sensitive part of the zooids 

 as they are carried to the shaded side of the colony (see 

 Chapter VII), and the greater increase in activity of the 

 zooids on the shaded side is no doubt due to this sudden 

 decrease of intensity, while the activity of all the zooids 

 is probably augmented by the effect of the continued 

 illumination. 



(7) An increase in light energy may produce the same 

 effect on reactions as a decrease in heat energy. Chla- 

 mydomonas, for example, becomes negative in constant 

 temperature if the light intensity is increased or in con- 

 stant illumination if the temperature is decreased. (See 

 Chapter XIII). 



(8) Acids, certain narcotics and salts, and at least one 

 alkali, may cause a change in the sense of reaction from 

 negative to positive in Gammarus pulex. Any condition 

 which acts as a depressant may cause Ranatra or Arenicola 

 larvae to become negative. 



(9) The stimulating effect of the different rays in the 

 spectrum is specific. But it is not the same in all organisms. 

 With a given amount of energy some are most strongly 



