Theories of Evolution 7 



only a sieve, which decides what is to live, and 

 what is to die. But evolutionary lines are of 

 great length, and the evolution of a flower, or of 

 an insectivorous plant is a way with many side- 

 paths. It is the sieve that keeps evolution on 

 the main line, killing all, or nearly all that try 

 to go in other directions. By this means nat- 

 ural selection is the one directing cause of the 

 broad lines of evolution. 



Of course, with the single steps of evolution 

 it has nothing to do. Only after the step has 

 been taken, the sieve acts, eliminating the unfit. 

 The problem, as to the manner in which the in- 

 dividual steps are brought about, is quite an- 

 other side of the question. 



On this point Darwin has recognized two pos- 

 sibilities. One means of change lies in the sud- 

 den and spontaneous production of new forms 

 from the old stock. The other method is the 

 gradual accumulation of those always present 

 and ever fluctuating variations which are in- 

 dicated by the common assertion that no two 

 individuals of a given race are exactly alike. 

 The first changes are what we now call ' ' muta- 

 tions," the second are designated as " individ- 

 ual variations, ' ' or as this term is often used in 

 another sense, as " fluctuations." Darwin rec- 

 ognized both lines of evolution; Wallace disre- 

 garded the sudden changes and propQsed fluctu- 



