THE EVOLUTION OF INSTINCT 131 



attempted to show that the main factor to which instincts 

 owe their origin is natural selection. The necessity for an 

 appeal to a previous intelligence was swept away, and while 

 Darwin did not deny that the inheritance of acquired char- 

 acters played a part in the development of instinct, he 

 ascribed to this factor a very subordinate role, and, as we 

 have seen, pointed out some very serious objections which 

 beset the theory. 



Darwin's theory assumes that instincts vary. Animals 

 which are endowed with congenital variations of instinct 

 which are advantageous to them will, other things equal, 

 survive; those w r hich have injurious variations will tend to 

 perish. Fortunate variations of behavior may thus be 

 accumulated along useful lines and build up complex and 

 highly adaptive modes of behavior. That, as the theory 

 requires, instincts, like corporeal structures, are subject to 

 congenital variations we have abundant evidence. We should 

 of course expect that variations in instinct would follow 

 variations in structure, but we should scarcely expect from 

 this standpoint to find instincts so variable as they are. 

 Apparently a slight structural variation may produce a 

 variation in instinctive behavior that seems out of all pro- 

 portion to the cause. 



Some of the most careful investigations of variations of 

 instinct have been made by the Peckhams in their classical 

 studies on the instincts of solitary wasps. Concerning 

 Ammophila which stores its nest with caterpillars which 

 it paralyzes by stinging them in the ventral ganglia, the 

 Peckhams remark: "In the three captures that came 

 under our observation, all the caterpillars being of the same 

 species and almost exactly of the same size, three different 

 methods were employed. In the first seven stings were 

 given at the extremities, the middle segments being left 



