236 BOTANY: PRINCIPLES AND PROBLEMS 



was the chief factor in convincing scientific men of the truth 

 of the evokitionary theory in general; and whatever we may think 

 today of the merits of some of Darwin's hypotheses, we recognize 

 that the thoroughness of his scientific work and its revolutionary 

 effect on all lines of biological thought entitle him to rank as the 

 first great evolutionist. 



Darwin based the theory of Natural Selection upon three main 

 facts: Variations and their inheritance; over-production of ofT- 



FiG. 134.— Charles Darwin, 1809-1882. 



spring with the consequent "struggle for existence"; and the 

 "survival of the fittest." 



He was vividly impressed by the occurrence of variations in all 

 animals and plants and studied them carefully, endeavoring 

 to discover their causes. Like other scientific men of his day, 

 Darwin did not clearly understand the mechanism and laws of 

 inheritance. He believed, at least in his earlier work, that 

 "acquired" characters may be transmitted to'offspring, but this 

 belief did not form the essential basis for his theory, as it did for 

 that of Lamarck. The main fact which he emphasized was that 

 variations in all directions are exceedingly abundant and that in 

 many cases they are certainly transmitted by inheritance to the 

 offspring. 



The overproduction of progeny in plants and animals forms the 

 next step in the theory. If all seeds which are produced were to 



