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 réle, 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 which 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 wehave abundant evidence. Weshould 
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 
