Seen from above, the herds formed by herd animals
have rounded forms, which, even during rapid movement,
maintain a minimal circumference and are reminiscent
of drops of mercury rolling over rough terrain.
The same is true of flocks of birds and shoals of fish
moving in space. Even during rapid movements, individuals
maintain the closest possible proximity. The outlying
flyers always press towards the centre. The flock
has a boundary layer, although this is not necessarily
distinct. The form of the flock seen as a whole is related
formally not only to soap bubbles in the wind, but also
to moving drops of oil floating in water (illus. 9).