EXTERNAL INFLUENCES AS CAUSES OF VARIATION 259 



Birds develop only at high temperatures. The normal tem- 

 perature for the chick is 38. Fere 2 incubated at temperatures 

 varying from 34 to 41. The individuals were all examined at 

 the same absolute time, and the following figures express the 

 percentages of development attained, taking that of 38 as the 

 standard : l 



The author remarks that some doubt attaches to the figures 

 under 35, 36, and 37, and calls our attention to the fact that 

 somewhere not far above 41 the series would become zero. 

 But for the range given, the development is in proportion to the 

 temperature, although the highest given (41) is considerably 

 above that attained under natural conditions. The growing chick 

 therefore does not, in nature, achieve its optimum. 



Effect of heat upon the direction of growth, thermotropism. 2 

 Without going into the methods of investigation, it appears that, 

 independently of the influence of light or other " tropisms," plants 

 are often positively or negatively thermotropic largely according 

 to temperatures. The plumule of seedling maize, for example, 

 is known to be positively thermotropic at ordinary temperatures, 

 while the radicle is positive between 15 and 35, indifferent at 

 37.5, and negative above. The indifferent point with the bean 

 (radicle) is given at 22.5. 



The' subject is little understood, and though the impulse of 

 thermotropism is weak as compared with that of heliotropism or 

 geotropism, it is supposed to be one which inclines the organism 

 to align itself in accord with the direction of heat rays, although 

 it is true that thermotropic plants are sensitive to conducted as 

 well as to radiant heat. 



Temperature limits of life. 3 All experiments indicate that as 

 the temperature rises above the maximum the first effect is heat 



1 C. B. Davenport, Experimental Morphology, Part II, p. 459- 



2 Ibid. pp. 463-467. 



3 For extended discussion, and for tables of temperature limits, see C. B. Daven- 

 port, Experimental Morphology, Part I, pp. 231-249. 



