VARIATION 



517 



attempt to explain it, his arguments for evolution were much 

 more convincing than those of his predecessors, and his explana- 

 tion was so well based upon facts and so well organized that it 

 was widely accepted. His book, the Origin of Species, published 

 in 1859, in which he set forth his argument for evolution and its 

 explanation, was based 

 upon 20 years of careful 

 observat : on, experiments, 

 and thought, and is one 

 of the greatest books ever 

 published. To this book 

 is largely due the accept- 

 ance of the doctrine of 

 evolution as based upon 

 natural selection, and 

 modern biology is said to 

 date from this book. The 

 two fundamental concep- 

 tions of this book are: 



(1) that the process of 

 creation is evolution; and 



(2) that the process of 

 evolution is based upon 

 natural selection. 



Natural Selection. - 

 According to Darwin's 

 theory of natural selec- 

 tion, the individuals of the plant and animal world are in- 

 volved in continuous competition in nature, and only those 

 best adapted to their surroundings survive. Thus through 

 the destruction of those individuals not able to survive in the 

 struggle for existence, a selective process, which permits only 

 certain individuals out of a large number to live and propa- 

 gate, is thereby established. Darwin* s theory of natural selec- 

 tion involves five fundamental conceptions, variation, inheri- 

 tance, fitness for environment, struggle for existence, and survival 

 of the fittest. 



Variation. Variation is the fundamental fact in Darwin's 

 theory of natural selection. By variations is meant the devia- 

 tions of organisms from a type chosen as a standard of compari- 



FIG. 464. Charles Darwin, the noted 

 scientist, to whose work the establishment 

 of the theory of evolution by natural selec- 

 tion is chiefly due. 



