So far, little has been said about change in the environment. The point of observation
could change and the source of illumination could change, but the streams did not flow,
the pebbles did not roll, the leaves did not fall, and the animals did not scurry about.
The environment has been described as shaped and textured and colored, as well as
illuminated by a moving sun, but as if frozen. Let us now bring the environment to
life. We need to consider a world in which events can happen.

Ecological events, as distinguished from microphysical and astronomical events,
occur at the level of substances, and the surfaces that separate them from the medium.
Substances differ in rigidity and thus in the degree to which their surfaces resist
deformation. Between the surfaces of clouds at one extreme and of solid rock at the
other are liquids, viscous substances, viscoelastic substances, and granular substances
whose surfaces are intermediate between these extremes in their resistance to defor-
mation. The reshaping of a surface requires force, the amount of force depending on
the substance.

It will also be remembered that substances differ in chemical inertness, or the
degree to which they resist reactions with agents like oxygen in the medium. The more
inert a substance is, the more its surface and its composition will tend to persist.
Substances also differ in their readiness to evaporate or sublimate, and this too affects
the persistence of their surfaces.

The distinction between objects that are attached to the ground and those that are
not should also be remembered in connection with ecological events. The detached
object can be moved without breaking the continuity of its surface with another surface,
but the attached object cannot. Note that an object can be resting on a surface of
support, in contact with it, without being attached to it. These distinctions will be used
in discussing motions as ecological events.

The laws of motion for bodies in space as formulated by Isaac Newton apply
only to idealized detached objects. The falling apple that, according to legend, hit Newton
on the head and led him to conceive the law of universal gravity was only an incident

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