exteroreceptors determine the activity but also, in some 
cases proprioreceptors. There may of course also be 
oroprioreceptive influences in some of the exteroreceptive 
examples such as those you suggest about head flagging and 
preening the back. 
About the manner of working of the pro nr i o re c e p t i ve or 
exteroreceptive influence I think it must be very mush as you 
suggest. I suppose that when an animal performs 
certain inertia. Let us say 
to go to a certain place and 
in the ground for worms. 
it has to overcome a 
to feed it has first 
bend down and search 
an activity 
that in order 
then to 
One would expect 
that if it is already in the feeding place it will experience 
a stronger tendency to feed than if it first has to fly there. 
And in the same wag if it is actually bending down close to the 
ground it may be more strongly aroused to feed than if it is 
standing up, (whether the influence If propriorecptive or 
exteroreceptive) . As you say, this influence mjj s t be some kind 
of feed-back:. So far so good. Xmacxxjuctkxtx But why should 
not the "extraneous activity" always ac cur whenever the 
particular posture is adopted. Does one in fact have to fall 
back on the idea of spark - over once again to ex plain t’^is ? 
I don't really know, but I not. Certainly I don t 
confine the idea, of postural influence on subsequent activities; 
to displacement activities. It has a much wider significance . 
Tor example I have seen in Eibl's hamster film how one of 
the hamsters made a typical back- stretching movement when the 
ben in the tunnel to its burrow made a sharp turn which so 
to speak started off the posture. Bill Ver plank also told me 
that postural influences are important in maze-running in rats. 
In short then it seems sure that CTX*KXx£xtH£iuBHXKXXKiu* 
a certain posture must increase the tendency of all acts 
(Russell et al . I) which have a similar posture. This increase 
in tendency must be some kind of feed-back process. Exactly 
how the tendency is increased I don't know. One situation 
where this fcxx&KXKyxKsqpxkaxx postural Influence may be 
important is inlselecting displacement activities as suggested 
by Bae rends (Tinberg, derived Activities), and by Lorenz in 
same year ( Proceedings of Deuts&fc., Zoolog . (resell. ). 
I wonder where you will be when this reaches you. We 
expect any day to hear any day that you have been expelled from 
some country or another as a counter-revolutionary - or eiected 
as president. Bae told me you would kindly keep an eye open 
for any cormorant display. The sort of thing I am interested 
to know id whether the body is tilted forward in the advertising 
display, and especially details of any wing movements which may 
accompany the displays, and of course any fuller accounts of 
the movements themselves. 
