FORTY-NINE GENERATIONS IN THE DARK. 1 89 



number of generations is large when compared with longer lived 

 animals. Forty-nine generations of mankind would cover about 

 fifteen centuries of time. 



The first changes noticeable in cave animals are the loss of 

 color and the degeneration of the eyes. So far, in the case of the 

 flies, no visible change in color has manifested itself. Sections 

 of the eyes showed all parts perfectly normal. Although I have 

 not succeeded in getting an accurate method of measuring it, 

 there is a noticeable difference in their reactions to light. These 

 flies are positively phototactic and if set free nearly always fly 

 toward the light. Those bred in the dark are still positive, but 

 they do not react so quickly nor do so many of them react. At 

 the end of the tenth generation, this difference was noticeable. 

 So noticeable, that when two vials, one containing flies bred in 

 the light and the other in the dark, were shown to several people 

 at the laboratory without them knowing what they were, they 

 immediately remarked that one lot went toward the light much 

 more quickly than the other. This tenth generation, after being 

 bred in the light for one generation, still showed a difference but 

 not quite so marked. Whether or not the flies of the forty-ninth 

 generation are more sluggish in their reactions to light than those 

 of the tenth generation is impossible for me to say as my method 

 of testing is not sufftciently accurate. I expect to continue the 

 experiment and hope later to devise a method by which I can 

 test the reactions of each fly individually and thus see whether 

 the effect of the darkness is a cumulative one. 



Much discussion has centered around the question of the 

 origin of cave faunas and several suggestions have been made in 

 regard to its solution, but it seems pretty generally concluded 

 that animals which now inhabit caves have entered them because 

 they were originally dark-seeking forms. The present experi- 

 ment shows that a positively phototactic animal might establish 

 itself in a cave if it were accidentally enclosed in such a place 

 and if a suitable food supply were present. 



Since no changes have fe^en produced in the color of the body 

 or in the structure of the eyes by breeding the fruit fly in the 

 dark for forty-nine generations, the question arises as to whether 

 the length of time which these flies have been bred in the dark 



