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 j|hown 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 



