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CAUSES OF VARIATION 



rays and to move toward or away from the source of light in 

 this direction." 



4. " Eyeless animals behave in this respect like animals hav- 

 ing eyes." 



III. " Heliotropic irritability of an animal manifests itself 

 frequently only at certain epochs of its existence." 



1-4. In winged ants this epoch is -at the nuptial flight; in 

 plant lice it is when the wings are present ; in the larvae of 

 Musca vomitoria negative heliotropism is most prominent when 

 the larva is fully grown ; and in a large number of animals the 

 irritability is opposite in the larval and in the adult stages. 



5. " Both night and day Lepidoptera are positively heliotropic, 

 and their heliotropism is similar to that of every other positively 

 heliotropic animal. The period of sleep of the night Lepidop- 

 tera, however, falls in the daytime, and only for this reason is 

 their heliotropism manifested exclusively at night." l 



IV. " In many animals heliotropic irritability is connected 

 with sexuality." Ants are sensitive only at the time of the 

 nuptial flight, and in both ants and Lepidoptera the males are 

 more sensitive than the females. 



V. " The behavior of an animal depends on the sum total of 

 its different forms of irritability." 



VI. Many animals are " compelled to orient their bodies 

 against the surfaces of other solid bodies," or to bring their 

 bodies " in contact with other solid bodies on as many sides 

 as possible (stereotropism)." 



VII. Animals " may be forced by light to move from diffused 

 light into sunlight and to remain exposed to the high tempera- 

 ture of the sunlight, even though it may cause their death." 



Considering all the " irritabilities " and " tropisms " to which 

 animals and plants are exposed and to which they react, it is 

 not necessary to appeal to instinct, or even to the nervous 

 system, to explain all movements of animals, nor is it well to 

 ascribe to them such anthropomorphic qualities as love of light, 

 distaste for darkness, preference, avoidance, curiosity, reason, or 

 other such bases of higher intelligent action. 



1 The question naturally arises, Why is the daytime the period for sleep in a 

 positively heliotropic animal ? The answer has not yet been given. 



