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THE AMERICAN NATURALIST [Vol. L 



normally begins degeneration at or before hatching and 

 is wholly absent from the week-old squab. It soon be- 

 came evident that the persistent ovaries were found prac- 

 tically exclusively in birds hatched from eggs of over- 

 worked series. Further study has shown in addition that 

 tlit'.v arise almost wholly from the eggs of autumn, and 

 predominantly then from the second egg of the clutch- 

 that is from eggs otherwise known to have the greatest 

 or strongest female-producing tendency. These ovaries 

 have sometimes weighed nearly a third as much as the 

 adult left ovary with which they were associated, and 

 have boon found in such birds dead at all periods from a 

 few days to fifteen months. We here attempt no ade- 

 quate description of this situation, but one can not have 

 observed the frequency of the persistence of this ovary 

 in the birds hatched from the eggs otherwise known to 

 be the most feminine from these overworked series with- 

 out conviction that the same pressure which carries the 

 eggs of spring from male-producing to female-produc- 

 ing levels, also carries the earlier female-producing level, 

 to another yet more feminine. 



In conclusion, the studies that have thus far been made 

 on sex, and on the experimental control of sex, in pigeons 

 go very far, we believe, toward an adequate demonstra- 

 tion that germs prospectively of one sex have been forced 

 to produce an adult of the opposite sex— that germs nor- 

 mally female-producing have, under experiment, been 

 made to develop into males ; and that germs which were 

 prospectively male-producing have been made to form 

 female adults. That neither selective fertilization, dif- 

 ferential maturation nor a selective elimination of ova 

 in the ovary can account for the observed results. Fur- 

 ther, and perhaps of more importance, these studies 

 throw much new light on the nature of the difference be- 

 tween the germs of the two sexes. This difference seems 

 to rest on modifiable metabolic levels of the germs ; males 

 arise from germs at the higher levels, females from the 

 lower; and such basic sex differences are quantitative, 

 rather than qualitative in kind. 



