EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



251 



to overcome an opposite but weaker one, in this case nega- 

 tive heliotropism. 



It is a noteworthy fact that irritability to light, while a prop- 

 erty of protoplasm in general, is more pronounced in some cases 

 than in others, even within the same organisms. This is true 

 not only in the eyes of animals, but, in general, the oral (anterior) 

 end of eyeless animals is much more sensitive to light than is 

 the aboral ; 1 as also is the dorsal surface more sensitive than 

 the ventral. Light therefore operates strongly to influence not 

 only the position but the locomotion of animals as well. 



The following conclusions from Loeb upon the influence of 

 light are valuable. 2 In substance they are : 



I. " The dependence of animal movements on light is in every 

 point the same as the dependence of plant movements on the 

 same source of stimulation." 



1. " The direction of the median plane or the direction of 

 the progressive movements of an animal coincides with the 

 direction of the rays of light." 



2. " The more refrangible rays of the visible spectrum are 

 more effective than the less refrangible rays." 



3. " Light of a constant intensity acts as a constant stimulant." 



4. Heliotropism is in a large measure dependent upon the 

 intensity of light, differing for different animals. 



5. " Heliotropic movements occur only between certain limits 

 of temperature." 



II. " The orientation of an animal toward a source of light 

 depends on the form of the body, just as the orientation of a 

 plant to light depends on the form of the plant." 



1. " Symmetrical points on the surface of dorsiventral animals 

 possess equal irritabilities." 



2. The " irritability of the oral pole of an animal is different 

 from the irritability of the dorsal pole," and is generally greater. 



3. " The irritability of the ventral surface is different from the 

 irritability of the dorsal surface." 



" These three conditions taken together cause dorsiventral 

 animals to place their median planes in the direction of the 



1 Loeb, Studies in General Physiology, pp. 



2 Ibid. pp. 81-84. 



