2 28 LIGHT AND THE BEHAVIOR OF ORGANISMS 



as they have learned to follow their mates or other objects 

 of interest. 



The fact that an insect flies into a candle flame at night 

 or a bird against a lighthouse tower and loses its life does 

 not indicate that its light reactions are not in general adap- 

 tive, as many assume. One might as well say, if a creature 

 in a room flies toward a window and is injured by striking 

 the glass, that its behavior is not adaptive. The environ- 

 ment in both cases contains artificial factors which animals 

 rarely experience. It remains for future investigators to 

 demonstrate whether or not insects and birds can learn 

 to avoid such pitfalls as candles and lighthouses. 



Whether or not the flight of birds against a lighthouse is 

 tropic, as Cole suggests in the following quotation, depends 

 upon the sense in which the term tropism is used: (1907, 

 p. 410) " The way in which migrating birds often, on stormy 

 nights, gather about lighthouses and dash into the glass only 

 to be killed, recalls strongly the flying of moths into a flame, 

 and it seems possible that this is an expression of photo- 

 tropism in birds which is ordinarily inhibited by other 

 responses." It seems probable that all creatures which fly 

 are guided on their course by sight at least with reference 

 to their immediate environment. It is well known that 

 birds seldom fly against the lighthouse windows unless the 

 night is dark and stormy. Under such conditions the light 

 intensity all about is very low, so that when the birds get 

 near the lighthouse they can see nothing but the light in it, 

 and if they are to fly toward something they can see, they 

 must evidently fly toward this light. If this is true the 

 factors involved in this phenomenon are similar to those 

 involved in regulating their flight toward any other object. 



5. General Summary and Conclusions 



The several summaries at the close of the different sec- 

 tions in this part and the table of contents should be re- 

 ferred to for a general idea of the subjects treated and the 



