External and Internal Factors in Evolution 319 
any change whatever in their condition, tend to vary; the 
kind of variation which ensues depending in most cases in a 
far higher degree on the nature of the constitution of the 
being, than on the nature of the changed conditions.” 
Most naturalists will agree, in all probability, with this con- 
clusion of Darwin’s. The examples cited in the preceding 
pages have shown that there are several ways in which the 
organisms may respond to the environment. In some cases 
it appears to affect all the individuals in the same way; in 
other cases it appears to cause them to fluctuate in many 
directions ; and in still other cases, without any recognizable 
change in the external conditions, new forms may suddenly 
appear, often of a perfectly definite type, that depart widely 
from the parent form. 
For the theory of evolution it is a point of the first impor- 
tance to determine which of these modes of variation has 
supplied the basis for evolution. Moreover, we are here 
especially concerned with the question of how adaptive vari- 
ations arise. Without attempting to decide for the present 
between these different kinds of variability, let us examine 
certain cases in which an immediate and adaptive response 
to the environment has been described as taking place. 
RESPONSIVE CHANGES IN THE ORGANISM THAT ADAPT IT 
TO THE NEw ENVIRONMENT 
There is some experimental evidence showing that some- 
times organisms respond directly and adaptively to certain 
changes in the environment. Few as the facts are, they 
require very careful consideration in our present examination. 
The most striking, perhaps, is the acclimatization to different 
temperatures. It has been found that while few active organ- 
isms can withstand a temperature over 45 degrees C., and that 
for very many 40 degrees is a fatal point, yet, on the other 
hand, there are organisms that live in certain hot springs 
