EXTERNAL INFLUENCES AS CAUSES OF VARIATION 249 



It is first of all dependent upon intensity of light, feeble illumi- 

 nation being ineffective, even if the characteristic rays be pres- 

 ent. Not only that, but some organisms are positively helio- 

 tropic in moderate light and negatively heliotropic in strong 

 light. In this way many fishes are held in a nearly constant 

 illumination, rising or falling in the water according to the 

 intensity of sunlight, and coming completely to the surface at 

 nightfall. 



Another element in heliotropism is temperattire, its influence 

 being most active at the maximum or normal, and lessening or 

 disappearing as it falls. 



Still another controlling influence is the condition of the 

 animal. Many caterpillars are positively heliotropic only when 

 unfed. They are thus led to ascend trees when hungry and to 

 descend when filled. 1 Still again, certain animals are heliotropic 

 only under peculiar circumstances ; for example, Loeb found that 

 winged ants exhibited no reaction to light except at the time of 

 their nuptial flight, when they were decidedly heliotropic. 2 



Influence of light upon the direction of growth. 3 Instances of 

 this influence upon the stems of plants are almost too common 

 to need mention. The leaning of plants toward the window, or 

 of trees over a stream, can be seen almost any day. 



It is noticeable that most leaves appear to be destitute- of 

 heliotropism, and yet it is not impossible to detect traces of its 

 influence. On careful observation it will be noted that some 

 leaves tend to present their upper surface at right angles to 

 light rays, while others tend to present the edge. 



It is a general principle of orientation that organisms with 

 radial symmetry, like most plants, present as nearly as possible 

 the end of the long axis to the source of light, with the lateral 

 parts equally lighted ; while organisms of bilateral symmetry, 

 like most animals, tend to present the dorsal surface at right 

 angles to the light, with the oral (head) end nearest its source, 

 the right and the left halves equally lighted, and the ventral 

 surface shaded. 



1 Loeb, Physiology of the Brain, p. 189. 



2 Loeb, Studies in General Physiology, pp. 52-53. 



3 C. B. Davenport, Experimental Morphology, Part II, pp. 437~445- 



