ADAPTATION AND AGGREGATION 243 



positive individuals the most sensitive part is then fully 

 exposed to the light, whereas in negative individuals it is 

 shaded. In sessile forms nothing more can be accomplished, 

 but in motile forms still more favorable conditions are ordi- 

 narily attained by locomotion after orientation, that is, 

 movement toward or from the source of greatest illumina- 

 tion, depending upon whether the organism is positive or 

 negative. 



The fundamental principle involved in orientation, as 

 shown in Part II, is the same in all organisms without 

 image-forming eyes, although the process differs much. Ori- 

 entation is the result directly or indirectly of the effect of 

 illumination on life processes. Movement toward unfavor- 

 able conditions generally induces orienting reactions. The 

 reaction leading to orientation is however frequently not a 

 response to an immediate unfavorable condition. It may 

 be a response to a sign quite as much as the sudden con- 

 traction which closes the valves of a mussel when a shadow 

 passes over it. The shadow in itself is of no particular con- 

 sequence to the mollusk, but what follows may be. Just so 

 the change of intensity has no particular influence on the 

 life processes of Euglena, e.g., but what follows may have. 

 Thus it is evident that orientation in the lower forms is 

 dependent upon the power of discrimination between dif- 

 ferent degrees of intensity — " Unterschiedsempfindlich- 

 keit " (Loeb), '* sensibilite differentielle " (Bohn). In many 

 of these forms orientation is undoubtedly, and in all it is 

 probably, a response to change of light intensity on some 

 part of the organism. At any rate it has in no instance 

 been demonstrated that it is, as Loeb states, ** a function 

 of the constant intensity," that orientation in light is like 

 orientation in an electric current. 



c. Orientation and extent of movement limited by 

 environment. — Many organisms orient and proceed toward 

 or from the source of light as far as the physical conditions 

 of the environment will permit and collect there. Areni- 

 cola and Eudendrium larvae, zoeae and many other aquatic 



