Theories of Evolution 7 



only a sieve, which decides which 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 how the individual steps are 

 brought about, is quite another side of the ques- 

 tion. 



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 * * individual 

 variations," or as this term is often used in an- 

 other sense, as ' ' fluctuations. ' ' Darwin recog- 

 nized both lines of evolution ; Wallace disregard- 

 ed the sudden changes and proposed fluctuations 



