MOLLUSKS, ARTHROPODS AND VERTEBRATES 227 



The fact that many animals with image-forming eyes 

 respond to size of the luminous area rather than to intensity 

 difference as shown by Parker (1903) and Cole (1907), is 

 not at all opposed to the idea of Holmes stated above. In 

 this fact we have an answer to the much-discussed and 

 perplexing question as to why moths fly toward a candle at 

 night and not toward the moon. The reason clearly is that 

 in moonlight there are large illuminated areas all about, 

 while in candle light the objects about are so faintly illumi- 

 nated that the moths do not react to light reflected from 

 them. But this answer is clearly superficial. The question 

 is: Why do the Mourning-cloak butterflies, e.g., fly toward 

 large illuminated patches rather than toward the sun, which 

 is much brighter ? Parker (1903, p. 465) says it is because 

 the larger area makes a larger " spot on the retina." " I 

 therefore believe that Vanessa antiopa stays near the ground 

 on bright sunny days because its flight is directed by large 

 bright retinal spots rather than by small ones, even though 

 the latter are of vastly greater intensity." These answers 

 are no doubt correct, but they are not fundamental. The 

 reactions referred to above are in general adaptive, and an 

 explanation of them must be sought along the same general 

 lines as an explanation for any other question involving 

 adaptation. It is frequently said that organisms in water 

 are limited in their movements toward the sun by the sur- 

 face of the water, but that insects flying in air are not thus 

 limited. Referring to Labidocera Parker says (p. 462): 

 "Their positive phototropism is held in check by their inabil- 

 ity to pass above the surface of the water. No such barrier 

 holds the butterfly to the earth." It is evident that this is 

 true only in a restricted sense. Insects flying up in the air 

 soon find their limit, and of course no one knows how many 

 have tried this very thing during the process of fixing the 

 adaptive habit of reacting positively to large luminous 

 areas rather than to small ones of much higher intensity. 

 Thus it may be that they have learned to go toward the 

 larger luminous areas in preference to smaller ones much 



