External and Internal Factors in Evolution 321 



periments on flagellate protozoans. He kept them in a warm 

 oven, beginning at first at a temperature of 16.6 degrees C. 

 " He employed the first four months in raising the tempera- 

 ture 5.5 degrees. This, however, was not necessary, since 

 the rise to 21 degrees can be made rapidly, but for success 

 in higher temperatures it is best to proceed slowly from the 

 beginning. When the temperature had been raised to 23 

 degrees, the organisms began dying, but soon ceased, and 

 after two months the temperature was raised half a degree 

 more, and eventually to 25.5 degrees. Here the organisms 

 began to succumb again, and it was necessary repeatedly to 

 lower the temperature slightly, and then to advance it to 

 25.5 degrees, until, after several weeks, unfavorable appear- 

 ances ceased. For eight months the temperature could not 

 be raised from this stationary point a quarter of a degree 

 without unfavorable appearances. During several years, 

 proceeding by slow stages, Dallinger succeeded in raising 

 the organisms up to a temperature of 70 degrees C, at which 

 the experiment was ended by an accident." 1 



Davenport and Castle carried out a series of experiments 

 on the egg of the toad, in which they tried to acclimatize 

 the eggs to a temperature higher than normal. Recently 

 laid eggs were used; one lot kept at a temperature of 15 

 degrees C, the other at 24-25 degrees C. Both lots de- 

 veloped normally. At the end of four weeks the tempera- 

 ture point at which the tadpoles were killed was determined. 

 Those reared at a temperature of 15 degrees C. died at 41 

 degrees C, or below; those reared at 24-25 degrees C. sus- 

 tained a temperature 10 degrees higher; no tadpole dying 

 in this set under 43 degrees C. " This increased capacity 

 for resistance was not produced by the dying off of the less 

 resistant individuals, for no death occurred in these experi- 

 ments during the gradual elevation of the temperatures in 

 the cultures." The increased resistance was due, therefore, 

 1 Quoted from Davenport's " Experimental Morphology." 



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