152 



EVOLUTION AND ANIMAL LIFE 



their issuance as active, winged creatures, are not of sufficient advan- 

 tage or disadvantage to the individuals to lead to a weeding out by 

 death or saving of such varying individuals by immediate selective 

 action. Whatever the rigor and danger of the outdoor bee life, these 

 variations seem to be insufficient to cut any figure in the persistence or 

 nonpersistence of any individual in the face of this rigor. 



"A case which really 

 seems to illustrate deter- 

 minate variation is that of 

 the variation of the flower 

 beetle, Diabrotica soror (Fig. 

 75). Among a thousand in- 

 dividuals collected on the 

 University campus in 1895, a 

 certain condition of variation 

 in the elytral pattern exists, 

 as represented graphically by 

 Fig. 90. In 1901 and 1902, 

 other thousands collected 

 from the 'same place and 

 examined to determine the 

 condition of the variation in 

 this pattern, show a dis- 

 tinctly different status, as il- 

 lustrated in Figs. 91 and 92. 

 (To be sure that a series of 

 a thousand individuals really 

 reveals the conditions of this 

 pattern variation, repeated 

 series of 1,000 individuals 

 each were examined and 

 found practically identical.) 

 The difference in the varia- 

 tion status between the 1895 lot and the 1901-2 lots consists in the 

 dominance in 1901-2 of one of the two 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 ? 



FIG. 90. Frequency polygon of variation of 

 elytral pattern in 905 specimens of the Cali- 

 fornia flower beetle, Diabrotica soror, collected 

 at Stanford University, 1895. (After Kellogg 

 and Bell.) 





