898 The American Naturalist. [November, 
were able to endure it, until a final temperature of 158° was 
reached, in the course of seven years, at which there appeared 
to be a perfect adjustment of their vital activities to the abnor- 
mal environment. 
There were critical periods as the temperature was increased, 
at which a considerable time was required for the organisms to 
become fully acclimated, and when this was secured, a more 
rapid increase of temperature was for a time admissable, until 
another point was reached at which a further rise in tempera- 
ture could not for some time be made. 
No advance was possible for eight months after the tempera- 
ture of 78° was reached; at 93° a halt of nine months was 
required; and at 137° a further increase of temperature was 
not permitted until after twelve months had elapsed. The 
manner in which the organisms were affected at the critical 
periods will be sufficiently illustrated by Dr. Dallinger’s 
remarks on their behavior at 137°. He says, “ when the 136th 
degree had been passed there were symptoms of oppression 
and distress, and on touching 137° this was very manifest, ” 
and it was found necessary “to play the thermal point back- 
wards and forwards for three weeks before there was an 
approach to normal activity and fecundity.” At the close of 
the 12 months, during which the temperature was maintained 
at 137°, there was an increase in the vacuolation of the proto- 
plasm, which disappeared on raising the temperature 4° in 
the following month. From this time more rapid progress 
was made until the final temperature of 158° was reached, 
when the experiment was terminated by an accident to the 
apparatus. 
At times a slight increase of temperature was not tolerated 
until the changed habits of their protoplasm provided for the 
complete adjustment of their vital activities to the new envi- 
ronment, but when this adaptation was fully attained there 
was apparently developed an increased flexibility of their 
organization that enabled them for a time to bear a compara- 
tively rapid rise of temperature without any perceptible dis- 
comfort, but a limit to this toleration was again soon reached. 
The organisms that had been trained to live at a temperature 
