104 AN INTRODUCTION TO SCIENCE 



When the question is looked at in its broadest aspects it 

 is evident that since the world became habitable the condi- 

 tions of existence have undergone incessant change. Liv- 

 ing things have changed. Collectively, they have continu- 

 ously adapted themselves to their environment. Therefore, 

 whatever may be the proximate cause of their variability, it 

 is ultimately due to the action of the environment. 



