VARIATION AND MUTATION 



153 



"When one straightens up after a careful microscopic examina- 

 tion of the pattern of Diabrotica to determine its variation, one 

 is sure that no other enemy of these flower beetles can be conceived 

 to use such discrimination as ours. Does the fly catcher swoop- 

 ing from its station on fence post or tree branch determine which 

 of two heavily flying Dia- 

 broticas shall be its prey on 

 the basis of 'two middle 

 spots on left elytron partial- 

 ly fused' in one and 'these 

 two spots not touching' in 

 the other? To our minds 

 the change in variation 

 status, the dominance of one 

 mode to-day which was the 

 subordinate mode in 1895, 

 is not due to the action of 

 selection. We do not indeed 

 hesitate to believe in those 

 'unknown factors of evolu- 

 tion' which may produce, 

 among other results, that 

 condition of affairs best 

 najned ' determinate varia- 

 tion.' This variation is not 

 necessarily to be conceived 

 of as purposeful or even 

 advantageous ; if by its 

 cumulation it becomes a 

 disadvantage of life - and - 

 death value, natural selec- 



FIG. 91. Frequency polygon of variation of 

 elytral pattern in 905 specimens of the Cali- 

 fornia flower beetle, Diabrotica soror, col- 

 lected at Stanford University, October, 1901. 

 (After Kellogg and Bell.) 



tion, which is after all a 

 logical necessity and un- 

 doubtedly an actual actively regulative factor in species control, will 

 take care of it." 



In the light of the foregoing discussion of the categories and 

 characters of variations, it is obvious that a well-grounded 

 knowledge of variability and variations, a knowledge based on 

 careful extensive statistical and experimental studies, is essen- 

 tial as a basis for any effective investigation of the factors and 



