GENERAL CONSIDERATIONS 359 



clear off the ground, is evidently pure instinct. Wasps 

 of some species are persistent in making a final examination 

 of the interior of the burrow before taking in the prey; 

 other species are as determined upon plunging into the bur- 

 row with their burden immediately upon their arrival at 

 the nest. Instinctive behavior is exhibited by Scotia dubia 

 in that it flies only about the barn where its prey can be 

 found. In the case of Alyson melleus, which uses delicate 

 leaf -hoppers that easily become dry and hard, the habit 

 of nesting always in moist places now seems general for 

 this species. 



The above cases are only a few instances from the long 

 list of instinctive activities of wasps which might be men- 

 tioned. While all of these habits are, in some such form, 

 either necessary or advantageous to the well-being of the 

 species, most of them would admit of minor digressions 

 individual variations, if you please in the way of carry- 

 ing them out or of doing them under the difficulty of ab- 

 normal conditions. Many such deviations have been dis- 

 covered in our field work; more of them are ferreted out, 

 of course, with an increased scope of the number of obser- 

 vations. Almost more interesting and important than the 

 instincts themselves are the variations and modifications 

 of the usual habit. These details of the behavior of mem- 

 bers of a species, these modifications, slight though they 

 may appear, are probably the handle upon which Natural 

 Selection works the evidence of evolution in some form 

 in progress before our eyes. Among these variations in 

 instinct which we have fortunately espied in wasp life we 

 may cite the following from the preceding pages : 



Bembix nubitipennis occasionally rob one another of their 

 prey, when instinctively they ought to find their own pro- 

 visions. 



While nest-building is probably instinctive, we have 



