DARWIN 



ance of a few species, but the great majority of cases 

 must have been due to the unknown laws which gov- 

 ern fecundity and sterility. Striking examples of great 

 fluctuations in numbers are seen frequently in the 

 sudden appearance of great swarms of insects which 

 reappear for two or three successive years and then 

 cease, although there has been no multiplication of 

 enemies, no lack of food, and no change in environ- 

 ment. 



The Darwinian theory of evolution, as it stands 

 today with the modifications which have been made 

 since the time it left Darwin's hands, is a complex 

 doctrine which involves several ideas whose relative 

 importance has not been specified. There are first, 

 the general postulates of heredity common to all the- 

 ories of evolution; organisms tend to vary; offspring 

 are always different from their parents; and, as a gen- 

 eral rule, variation tends to increase the differentia- 

 tion and complexity of the organism. In addition, 

 there are the specific ideas which distinguish natural 

 selection from other causes for variation. 



There has been a vast deal of discussion about the 

 general laws of heredity. Biologists evade the state- 

 ment that the organism has an innate tendency to 

 vary, and attempt to account for variation by phys- 

 ical and chemical forces from a study of the cell and 

 the embryo. But the fact remains, except for a few 

 minor facts of hereditary changes, we are absolutely 

 unable to predict what variations will occur in future 



C 209 ] 



