328 KELLOGG AND BELL 



modal conditions found to exist in the species, which in 1895 

 was not the dominant one. There has been a marked change 

 in seven years, not in the pattern itself but in the prevalence or 

 dominance of one type of pattern/ Has the change been 

 brought about by natural selection? Or is it the result of a 

 determinate variation caused by we know not what intrinsic or 

 extrinsic factors ? The variation in Diabrotica's elytral pattern 

 is wholly comparable with the variation in Uippodaniia's elytral 

 pattern. The fusion, partial or complete, of two adjacent spots 

 produces Diabrotica's variety of pattern ; the suppression or 

 addition, partial or complete, of various spots produces Hippo- 

 damia's larger variety. But in Htppodatnia all the variety ex- 

 ists among individuals after exposure, for practically all of the 

 time that such exposure, will occur, to the rigor of selection 

 among individuals. Shall any greater effectiveness be ascribed 

 to this rigor in the case of Diabrotica than actually exists in the 

 case of Hippodamia'^. 



When we straighten up after a careful microscopic examina- 

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

 assure ourselves that no other enemy of these flower-beetles can 

 be conceived to use such discrimination as ours. Does the fly- 

 catcher swooping from its station on fence post or tree branch 

 determine which of two heavily flying Diabroticas shall be its 

 prey on the basis of " two middle spots on left elytron partially 

 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 in evolution" which may 

 produce among other results that condition of affairs best named 

 " determinate variation." 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 selection, which is after all a logical necessity and un- 



^ Moreover, in a 1904 lot, consisting of the same number of individuals and 

 collected from the same locality as the lot of 1895 and 1901-02, there is (vpithin 

 2 years) a marked rise in the percentage of the class which, being the mode in 

 1901-02, was in the minority in 1895. 



