External and Internal Factors in Evolution 321 
periments on flagellate protozoans. He kept them ina 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.” 
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.” 
y 
