140 THE AMERICAN NATURALIST [Vol. LI I 



that the cotton boll-weevil has a different zero or threshold 

 of development and different temperature optimum for 

 each humidity. 



(e) Oxygen Threshold 

 The development of various invertebrates is stopped by 

 insufficient oxygen (Loeb, '06, and citations). Johansen 

 and Krogh found that if the oxygen pressure was reduced 

 to one half by reducing the air pressure to 380 mm. of 

 mercury development of plaice eggs was retarded. The 

 oxygen pressure threshold of development lies below the 

 amount which will go into solution from air at pressure 

 of 230 mm. of mercury, but at this concentration much 

 care was necessary to keep the eggs alive. Shull ('11) 

 determined the oxygen minimum for the germination of 

 the seeds of Xanthium. 



(/) Light Threshold 

 Loeb ('11) states further that light is necessary to the 

 regeneration of zoids in Eudendrium. Its absence is 

 further known to slow development in larvae of insects 

 which normally live in the light (Bachmetjew, 692) . Smith 

 found that light accelerates the development of salmon. 

 Johansen and Krogh found little difference between ma- 

 rine fishes grown in light or in dark. Davenport ('99) 

 summarized the literature to that date and showed on the 

 authority of Yung that moderately strong light increased 

 growth. 



(g) Food Threshold 



Recent work has shown that food may be either qualita- 

 tively or quantitatively deficient and cause standstill in 

 the development of mammals. Thus Osborne and Men- 

 del (p. 101) show the following methods of producing it: 



(1) By under-feeding with rations of suitable qualita- 

 tive make-up; (2) by the use of diets containing an ade- 

 quate protein but with inorganic salts supplied in the form 

 of a mixture of pure chemicals together with sucrose and 

 starch as the carbohydrate component; (3) by restricting 



