INSTINCTIVE BEHAVIOUR IN INSECTS 71 
involving a larger measure of central (as opposed to local or 
ganglionic) co-ordination, and due to the more widely-spread 
effects of stimuli in which both external and internal factors 
co-operate, we shall probably get as near as is possible to the 
distinction of which we are in search. But it must be remem- 
bered that there are cases in which the distinction can hardly 
be maintained. 
We are now in a position to define instinctive behaviour 
as comprising those complex groups of co-ordinated acts which 
are, on their first occurrence, independent of experience ; 
which tend to the well-being of the individual and the pre- 
servation of the race; which are due to the co-operation of 
external and internal stimuli ; which are similarly performed 
by all the members of the same more or less restricted group 
of animals; but which are subject to variation, and to sub- 
sequent modification under the guidance of experience. 
II.—InstIncTIVE BEHAVIOUR IN INSECTS 
Since instinctive behaviour is, by definition, independent 
of experience, and since the animals which act instinctively 
are also, in many cases, able to act intelligently, it is clear 
that, apart from hereditary variations, we must expect to find 
acquired modifications of instinct. As Huber said of bees, 
their instinctive procedure often indicates “a little dose of 
judgment.” It is, indeed, exceedingly difficult, as a matter of 
observation, to distinguish between hereditary variation and 
acquired modification. For the role played by these two 
factors in any given behaviour can only be determined if the 
whole life-history of the individual be known, and if there be 
opportunities for comparing it with the complete life-histories 
of other members of its race. And this is seldom possible. 
These considerations must be borne in mind as we proceed 
to a brief study of some of the instinctive modes of behaviour 
in insects. 
Dr. and Mrs. Peckham’s investigations on the instincts and 
