368 LIGHT AND THE BEHAVIOR OF ORGANISMS 



stimulated by blue, others by violet or ultra-violet, others 

 by green and yellow, and still others by red and infra-red 

 (see Summary to Part IV, Chapter XIX). 



(10) Reactions to light are variable, modifiable, and in 

 general adaptive, (a) An attached specimen of S ten tor 

 coeruleus, for example, may contract suddenly when light 

 of a given intensity is flashed upon it, or It may merely 

 swing about its point of attachment or it may not respond 

 at all. Hydroides may remain in its tube after stimulation 

 by a given decrease of intensity only a few seconds, or It 

 may remain for several minutes. This difference In re- 

 sponse to the same external conditions must be due to 

 internal factors, {h) Wherever there is a reaction to a 

 sign, it Is probable that the response to a given external 

 condition has been modified. For example, Euglena under 

 certain conditions responds to a very slight decrease In 

 light intensity on the colorless anterior end, which Is In 

 itself of no consequence to the organism, but this slight 

 decrease In illumination Is usually followed by a greater 

 decrease on the entire body if there Is no change In the 

 direction of locomotion, and It Is of course for the welfare 

 of the organism to prevent this. It Is probable that 

 originally no response was given until the injurious condi- 

 tion was realized. Many similar illustrations are found 

 in organisms that respond to shadows which announce 

 the approach of an enemy, {c) Adaptation and regulation 

 are striking characteristics in nearly all reactions to light. 

 The reactions are adaptive not only under constant con- 

 ditions, but also under varying conditions, for if the environ- 

 ment Is changed the reactions change to meet the demands 

 of the new circumstances. Jennings has well said (1906, 

 P- 338): "Regulation constitutes perhaps the greatest 

 problem of life. How can the organism thus provide for 

 its own needs? To put the question In the popular form, 

 How does it know what to do when a difficulty arises? 

 It seems to work toward a definite purpose. In other 

 words, the final result of its action seems to be present in 



